Move some logic from factories to CustomForce objects

[andrrizzi]

In our factories (alchemical and restraints in particular), we create a lot of different custom forces that serve different purposes, but to recover and manipulate those forces we currently have to rely on many implementation details to distinguish them from other forces (see for example AbsoluteAlchemicalFactory._find_force_components or the restraint unbiasing implementation in choderalab/yank#887).

I was thinking of expanding the forces module and encapsulate some of this logic into actual RestorableForces objects, similarly to what we do with integrators, to easily differentiate them by type and equip them with a useful interface. I think this will simplify a lot of the code inside our factories.