Remove solvent ions

I used @swails's references and made a list of common solvent types. @rmcgibbo: Let me know if the additions are up to your standards.
I was rather astonished by that as well when I first learned about the whole "early vs. late binding" for default arguments (the problem with any mutable type as a default argument).  As it stands, it's no problem right now because `exclude` is never modified inside the function so the 'gotcha' highlighted in the SO thread can't materialize.  But I agree that it's a recipe for a dangerous situation going forward.