API extension: mol.AddAtom(atom) is now mol.AddAtom(atom, bool forceNewId=false) (preserving old behavior)

This is needed as molA += molB previously modified the atom Ids in molB by setting them all to NoId (there was no other way to pass the information into AddAtom).

(Rather than extend the API, an alternative approach would be to restore the atom Ids in molB after calling AddAtom.)