Fixed pdb2gmx -vsite hydrogen -o conf.pdb

Generation of v-sites did not initialise the elem field of the t_atom
structs. Later when writing the output to PDB format, that elem is
used for the final column. Somehow, the memory in elem was
uninitialized at that point, despite the use of srenew. Oh for proper
constructors and copies!

I observed the problem with v-sites generated from a POPC lipid, but
presumably the underlying cause is common to all kinds of v-sites, so
I have filled the elem field for two other kinds of v-site
generation. I have no idea why not all of the aromatic v-site
generation schemes involve filling a new t_atom struct.

Change-Id: I69efee07d8fd51192808a85d5704d488a0d310be