Refactor: Maybe GW should inherit from FGW instead of vice versa

[selmanozleyen]

I was wondering if it would make more sense to inherit GWProblem from FGWProblem since FGW kind of includes GW in itself. This is a problem since when GW inherites prepare it removes the linear arguments: (xy_callback etc.). And because of this we need to use CompoundProblem.prepare instead of super().prepare which is not ideal if you ask me.

https://github.com/theislab/moscot/blob/d54060a83de43d779ac50e0125021f2904513e4d/src/moscot/problems/generic/_generic.py#L581-L592