Add "base" Settings classes

[IAlibay]

Following from today's discussion on where different utilities go, one thing that we can possibly more easily move forward with is the idea of moving base classes to gufe.

In different protocols, we have quasi empty base Settings (e.g. BaseSolvationSettings) that we then subclass to use more specifically in a Protocol (e.g. PackmolSolvationSettings). The use case here is that we can then have reusable methods shared between protocols that type a certain Setting but don't always have the same attributes
For example OpenMM AFEs share a protocol unit base class with ABC methods that are implemented later depending on the use case.

[IAlibay]

This would be very similar to the base errors Josh added recently.