[Bridges] pass variable names through ParameterToEqualToBridge

[odow]

Closes #2762

But I'm not 100% sure we should merge this. It's really just a single special case. It isn't obvious what naming convention we should use if the map is y := Ax + b.

[odow]

This is actually not as easy as I thought.

[odow]

I'm not convinced we should merge this, but if we close this, we should also close #2762

[joaquimg]

I like the behavior. This is extremely useful for debugging models with parameters.

[odow]

I'll let @blegat decide what we should do

[blegat]

What is this for ?

[blegat]

Maybe add a comment explaining

[blegat]

I think that in general, bridge could optionally implement names, just the same way they can optionally implement any attribute.
The bridge optimizer could check if they support constraint names (for constraint bridges) or variable names (for variable bridges) and if so, it will set it.
For SetMapBridge, we know variable bridges create a unique variable in the scalar case so it's clear we can pass the name of that variable. But actually, when there is a nontrivial mapping, we like to give it a different name so we actually don't really want to pass the name along.
So I think that we should only pass the name along if it's mapped identically so this PR seems to do exactly what we should do.