Fixed bug inverting compression precision.

In the TNG library the compression precision was modified to
suit GROMACS compression precision better (i.e. indicating
the multiplier applied to the compressed value). There was no
change in GROMACS to reflect this, meaning that compressed TNG
would have 0 coordinates for almost all atoms. That is fixed
in this commit.

Change-Id: Id906c710deb68d172a38acec34892aa4823c039d