solvate and handling charges methods in `ProteinMutationExecutor` should do the sanity checking themselves

[ijpulidos]

There might be some logic needed to check if it makes sense to solvate the system or not, but I think this should also happen inside _solvate() method Such that it warns (or errors out) if users are trying to solvate an "unsolvatable" system. Something similar should happen when charge changes occur, these should be dealt in the _handle_charge_changes() method.

Originally posted by @zhang-ivy in https://github.com/choderalab/perses/issues/1024#issuecomment-1148875313

[ijpulidos]

Related issue https://github.com/choderalab/perses/issues/1034

This one is aimed more towards having the sanity checkings inside the _solvate or _handle_charge_changes methods themselves, rather than implementing the sanity checks themselves.