Create a ProteinMapping subclass

[IAlibay]

To do protein-protein mapping, it would be good to have a mapping subclass that is specific to ProteinComponents (that way we don't expect folks to use LigandAtomMapping).

[ijpulidos]

I think this is still something we want to do, but so the current implementation relies on ComponentMapping in #106 . Something to revisit in future developments, surely.