Make 'gmx sasa' volume computation translation-invariant

Compute the center of the molecule also when PBC is used, instead of
using the origin as the center.  The center is used for volume
computation, and this makes the output translation-invariant with PBC.
This also makes the results of PBC and no-PBC computation match for the
same structure in case there are no contacts over the PBC.

The whole volume algorithm is broken if the molecule is split across
PBC in the input structure (which is likely the case if one wants to use
-pbc), and that would require substantially more effort (essentially,
making each continuous surface whole before computing the volume), so
only added a note to 'gmx sasa -h' to explain this for now.

Change-Id: I1cf75505ceb403f5650f05453ec2bc7b979d1e9e