Avoid some incorrect behavior with gmx solvate

Avoid some incorrect behavior with gmx solvate

gmx solvate cannot replicate non-rectangular solvent boxes correctly
(there are several different places that assume a diagonal box matrix),
so give a fatal error if that is attempted.  To support some uses with
triclinic boxes, skip the replication step if the solvent and target box
sizes are already equal.

Support for general triclinic boxes can be added separately, and the
check introduced here can be valuable even in that case: it keeps a
pre-equilibrated solvent box intact if the target box size is the same.

Related to #2148.

Change-Id: I078df1f2279ccb758b11787becb89f5dbbbdfca7