Fixed gmx rmsf -q -oq.

Fixed gmx rmsf -q -oq.

xref was used instead of pdbx in that case, which led to a PDB file
containing B-factors to have the coordinates based on those from the
-s file, rather than -q file. The tool description (and the other
logic) suggests that using -q should be the behaviour, in particular
because the contents of xref were then unused. gmx rmsf -oq was
otherwise fine.

Also fixed where xref was being abused as a temporary vector, which
asks for more such trouble, so improved that (without changing
functionality).

Noted TODO that npdbatoms might not match top.atoms.nr leading to
possible segfaults.

Change-Id: Id2c14fa5ca93c2a8de87f9bfa144a6db38fcc97f