Added Drude mode for adding ions.

Drude ions have the core ion and an associated Drude particle,
so the present version of genion allows for a multi-atom ion
insertion to account for this fact.  Atom names are enumerated
based on CHARMM nomenclature for ions.  This could, in theory,
be extended to include other force fields, as well.

Change-Id: Iba46599b7d3ee58f41a247356696774b511272f2