Fixed pdb2gmx bug in adding Drudes.

In the unusual case that the user supplies -ignh with an
input file that has Drudes already built, duplicate Drudes
would be added because they would no longer be immediately
after the parent heavy atom.  In most cases, if Drudes are
present, they will also have H built in (e.g. CHARMM-GUI or
something else), but we have to protect for this case.

The topologies will have atoms in different orders in these
cases, but there is no functional difference between them.

Change-Id: I4e394954ccbbdb52c2efcc47efa2f21c54b75f0f