Add a helper for consistently creating PV names

[GDYendell]

Currently PV names are formatted in multiple places and risk being inconsistent. There should be a single function that creates these PV names.

This may require refactoring the Attribute / Controller / Mapping structure to allow this function to have a simple interface. Currently the places that format PV names combine the path from the controller with the name of the attribute from the mapping.

Additionally the GUI needs to manually add the prefix, whereas the IOC does builder.SetDeviceName, so the prefix should probably be an attribute of EpicsGUI and applied while making the pvi signals to make this consistent.

[GDYendell]

We don't call builder.SetDeviceName as of #54, so this should be slightly simpler.