Add terminal support

[Tc-Fast]

This draft adds support for FMI 3.0-style terminals to SSP 2.1. The draft closely follows the proposals from @DagBruck in #83.

Related to #59, #56, #84

Terminals are defined globally at the SSD root level (similar to Units and Enumerations), giving them a name that components can reference by. A terminal connector is declared with kind="terminal" and carries a <ssc:Terminal name="..."/> child pointing at the global terminal defintiion.

Member variables are bound by naming convention: a connector named portName.memberName is automatically associated with the terminal connector portName. This avoids any new connection syntax and keeps full backward compatibilty with SSP 2.0 systems.

Terminals carry a matchingRule attribute (plug, bus, sequence, none) taken from the FMI 3.0 terminal semantics, which tools can use for connection validation. Fan-out of terminal connections follows the same rules as scalar connections — an input member may only be written by one source.

┌──────────────────────────────────────────────────────────────────┐
│ System                                                           │
│  ┌───────────────┐                        ┌────────────────┐     │
│  │ Controller    │                        │ Plant          │     │
│  │               │                        │                │     │
│  │  controlPort <╪═══╦════════════════════╪> controlPort   │     │
│  │               │   ║                    │                │     │
│  │ externalSet- <┼───║────────────────────┼  measuredPos.  │     │
│  │  point        │   ║                    │                │     │
│  └───────────────┘   ║                    └────────────────┘     │
│                      ║                                           │
│                      ║     ┌──────────────────────────────────┐  │
│                      ║     │ Monitor                          │  │
│                      ║     │                 ┌──────────────┐ │  │
│                      ║     │                 │ Logger       │ │  │
│                      ╚═════╪> monitorPort    │              │ │  │
│                            │   .position ────┼> logPosition │ │  │
│                            │   .velocity ────┼> logVelocity │ │  │
│                            │                 └──────────────┘ │  │
│                            └──────────────────────────────────┘  │
└──────────────────────────────────────────────────────────────────┘

═══  Terminal connection (single connection, bidirectional signals)
───  Scalar connection

<?xml version="1.0" encoding="UTF-8"?>
<ssd:SystemStructureDescription
    xmlns:ssd="http://ssp-standard.org/SSP1/SystemStructureDescription"
    xmlns:ssc="http://ssp-standard.org/SSP1/SystemStructureCommon"
    name="TerminalExample" version="2.1">

    <ssd:System name="System">
        <ssd:Elements>
            <!-- Controller: outputs position/velocity commands, receives force feedback -->
            <ssd:Component name="Controller" source="resources/Controller.fmu">
                <ssd:Connectors>
                    <ssd:Connector kind="terminal" name="controlPort">
                        <ssc:Terminal name="com.example.ControlSignals"/>
                        <ssd:ConnectorGeometry x="1" y="0.5"/>
                    </ssd:Connector>
                    <ssd:Connector kind="output" name="controlPort.position">
                        <ssc:Real unit="m"/>
                    </ssd:Connector>
                    <ssd:Connector kind="output" name="controlPort.velocity">
                        <ssc:Real unit="m/s"/>
                    </ssd:Connector>
                    <ssd:Connector kind="input" name="controlPort.force">
                        <ssc:Real unit="N"/>
                    </ssd:Connector>
                    <ssd:Connector kind="input" name="externalSetpoint">
                        <ssc:Real/>
                    </ssd:Connector>
                </ssd:Connectors>
                <ssd:ElementGeometry x1="-80" y1="-20" x2="-40" y2="20"/>
            </ssd:Component>

            <!-- Plant: receives position/velocity commands, outputs force measurement -->
            <ssd:Component name="Plant" source="resources/Plant.fmu">
                <ssd:Connectors>
                    <ssd:Connector kind="terminal" name="controlPort">
                        <ssc:Terminal name="com.example.ControlSignals"/>
                        <ssd:ConnectorGeometry x="0" y="0.5"/>
                    </ssd:Connector>
                    <ssd:Connector kind="input" name="controlPort.position">
                        <ssc:Real unit="m"/>
                    </ssd:Connector>
                    <ssd:Connector kind="input" name="controlPort.velocity">
                        <ssc:Real unit="m/s"/>
                    </ssd:Connector>
                    <ssd:Connector kind="output" name="controlPort.force">
                        <ssc:Real unit="N"/>
                    </ssd:Connector>
                    <ssd:Connector kind="output" name="measuredPosition">
                        <ssc:Real unit="m"/>
                    </ssd:Connector>
                </ssd:Connectors>
                <ssd:ElementGeometry x1="40" y1="-20" x2="80" y2="20"/>
            </ssd:Component>

            <!-- Monitor subsystem: terminal on its interface, scalar access inside -->
            <ssd:System name="Monitor">
                <ssd:Connectors>
                    <ssd:Connector kind="terminal" name="monitorPort">
                        <ssc:Terminal name="com.example.ControlSignals"/>
                        <ssd:ConnectorGeometry x="0" y="0.5"/>
                    </ssd:Connector>
                    <ssd:Connector kind="input" name="monitorPort.position">
                        <ssc:Real unit="m"/>
                    </ssd:Connector>
                    <ssd:Connector kind="input" name="monitorPort.velocity">
                        <ssc:Real unit="m/s"/>
                    </ssd:Connector>
                </ssd:Connectors>
                <ssd:ElementGeometry x1="40" y1="-60" x2="80" y2="-30"/>
                <ssd:Elements>
                    <ssd:Component name="Logger" source="resources/Logger.fmu">
                        <ssd:Connectors>
                            <ssd:Connector kind="input" name="logPosition">
                                <ssc:Real unit="m"/>
                            </ssd:Connector>
                            <ssd:Connector kind="input" name="logVelocity">
                                <ssc:Real unit="m/s"/>
                            </ssd:Connector>
                        </ssd:Connectors>
                    </ssd:Component>
                </ssd:Elements>
                <ssd:Connections>
                    <!-- Inside the subsystem: connect individual terminal members -->
                    <ssd:Connection startConnector="monitorPort.position"
                                    endElement="Logger" endConnector="logPosition"/>
                    <ssd:Connection startConnector="monitorPort.velocity"
                                    endElement="Logger" endConnector="logVelocity"/>
                </ssd:Connections>
            </ssd:System>
        </ssd:Elements>

        <ssd:Connections>
            <!-- Terminal connection: carries position+velocity OUT and force IN -->
            <ssd:Connection startElement="Controller" startConnector="controlPort"
                            endElement="Plant" endConnector="controlPort"/>

            <!-- Same terminal to Monitor -->
            <ssd:Connection startElement="Controller" startConnector="controlPort"
                            endElement="Monitor" endConnector="monitorPort"/>

            <!-- Scalar connection (non-terminal) -->
            <ssd:Connection startElement="Plant" startConnector="measuredPosition"
                            endElement="Controller" endConnector="externalSetpoint"/>
        </ssd:Connections>
    </ssd:System>

    <ssd:Units>
        <ssc:Unit name="m">
            <ssc:BaseUnit m="1"/>
        </ssc:Unit>
        <ssc:Unit name="m/s">
            <ssc:BaseUnit m="1" s="-1"/>
        </ssc:Unit>
        <ssc:Unit name="N">
            <ssc:BaseUnit kg="1" m="1" s="-2"/>
        </ssc:Unit>
    </ssd:Units>

    <ssd:Terminals>
        <ssd:Terminal matchingRule="plug" name="com.example.ControlSignals"
                      terminalKind="com.example.ControlSignals">
            <ssd:Connectors>
                <ssd:Connector kind="local" name="position">
                    <ssc:Real unit="m"/>
                </ssd:Connector>
                <ssd:Connector kind="local" name="velocity">
                    <ssc:Real unit="m/s"/>
                </ssd:Connector>
                <ssd:Connector kind="local" name="force">
                    <ssc:Real unit="N"/>
                </ssd:Connector>
            </ssd:Connectors>
        </ssd:Terminal>
    </ssd:Terminals>

    <ssd:DefaultExperiment startTime="0.0" stopTime="10.0"/>

</ssd:SystemStructureDescription>