Added dummy mass for charmm TYR aromatic vsites

pdb2gmx was not able to create aromatic vsites
for charmm27 since this dummy mass center was missing.
Note that aromatic vsites are still largely untested,
and not recommended for general usage.

Fixes #587.

Change-Id: Ic7d2976536c6269d1dcf12b7d33bddab096c353f