Add convenience method to get the difference in formal charge between two SmallMoleculeComponents

[IAlibay]

This is an operation we have to do quite often in Protocols. It might be good to just have that as a convenience method?

[mikemhenry]

Do you have an API proposal? I think it would be crazy but we could define that if get the difference between two SmallMoleculeComponents you get a charge:

formal_charge_diff = gufe.SmallMoleculeComponent -  gufe.SmallMoleculeComponent

Or would it be like

gufe.SmallMoleculeComponent.get_charge_diff(other)

[IAlibay]

The get_charge_diff would be good. Otherwise even just some method in utils would work. I just noticed when working on the SepTopProtocol that it was a pretty handy thing to have around but it ended up getting duplicated in a few places.