Support of Modelica_StateGraph2 Connections.uniqueRoot

[HansOlsson]

In progress. The operator definitions need more work.

[HansOlsson]

This is now more or less the text from Modelica_StateGraph2; except that in case someone connects two unique roots tools are not restricted to using one message in the diagnostics. (The messages will normally be the same, but given that it is even an error to connect two roots with the same message it doesn't make sense to require an additional diagnostics in case they are different.)

[adrpo]

Looks good to me.