Fix selection of xtc groups not starting at index 0

The code to select atoms to write to the xtc file
only looped over the number of atoms selected for
output rather than all atoms. This meant that xtc
group selections that were not located at the start
of the atom indices would not be written correctly.

Fixes #1561.

Change-Id: I48886a75c682e38122dadcb2f752507b14b308fe