Support replacing solvent in insert-molecules
Make it possible to specify the solvent (or other set of atoms) with
-replace (as a selection) for gmx insert-molecules, and make the tool
replace residues from this set with the inserted molecules, instead of
not inserting there. It is assumed that the solvent consists of
single-residue molecules, since molecule information would require a tpr
input, which might not be commonly available when preparing the system.
Add a basic test for the functionality.
Change-Id: I3a60cdcec9a1675a116c13d83465b07a69b3388d