Add support to write out rigid water for GROMACS

[daico007]

This stem from a discussion with Dr. Potoff, we probably want to add the support to write out rigid water (only) to gromacs top file soon. Specifically, writing out the settles section so that we can have rigid water when it's in a mixed system.

[daico007]

GROMACS documentation for reference: https://manual.gromacs.org/current/reference-manual/topologies/constraint-algorithm-section.html#constraintalg