Improve PDB I/O (keep occupancy, b-factor, element)

Retain the extra PDB info such as occupancy, b-factors,
and element name columns with trjconv when providing a
PDB structure file. If you use a TPR file, we still
write the (given atomnumbers being present).
However, we deliberately don't store the pdbinfo in the
TPR file since that is a specification for a particular
experiment, rather than something that's valid in the
simulation.

Fixes #917, #1307.

Change-Id: I80cb8f5a250ba094fe81f32c58b4eb0298164053