Improve frame time/step handling in trjconv

Improve frame time/step handling in trjconv

Store the exact step in PDB/GRO file headers,
and be more careful about not claiming to have
time or step information when it was not available.
This change will avoid some of the problems
described in #2189, but it does not yet properly
fix the issue in the TNG library.

Refs #2189.

Change-Id: I44bb59fbca83da53f6e8d4e494ae6476a82eb7cd