Overconstrained connectors not handled by the NF

[Francesco Casella]

Overconstrained connectors are still not handled by the NF. This invalidates most MultiBody models, as well as other libraries (like PowerSystems) that rely on this feature.

This is a well-known issue, see #4138, just adding it as a separate ticket for the record.

[Adrian Pop]

Fixed with PRs:
​https://github.com/OpenModelica/OMCompiler/pull/2578
​https://github.com/OpenModelica/OMCompiler/pull/2579

[Adrian Pop]

I have been trying not to redo some of the things when handing the overconstrained connection graph but unfortunately that was not easily possible. I think we should expand the connectors once and then reuse the expansion in several of the phases from NFFlatten.flatten.
As is currently implemented I need to do the connector expansion several times:

1. for retrieving the actual component references of the overconstrained components in the connect, this is needed for typing the zeros(:) = equalityConstraint(A.OCGcomp, B.OCGComp);. This has to happen during typing.
2. again for retrieving the actual component references of the overconstrained components in the connect for building the graph, during flattening.
3. when handling the connection sets, this is done by default

Maybe we could expand the connectors once and store them in the Equation.CONNECT and don't do it again for 2) and 3).

@perost: what do you think about this?