Improve MotorAxis module

[falkoschindler]

The initial PR #15 only covered the most important feature of directional limit switches. But we would like to extend the MotorAxis module with the following features:

• allow using inverted switches
• support one or two switches
• reference() with reference switch: find reference switch with predetermined speed, send message to RoSys
• approach reference again more slowly?
• set the internal position to zero when referenced?
• handle the conversion between motor and output

[pascalzauberzeug]

PR #38 and #39 implement invertable switches