First run at a WIP Martini implementation

[andrewtarzia]

This is a first attempt at using Martini beads in CGExplore.

This adds the ability write and read .itp files using https://github.com/maccallumlab/martini_openmm (a new dependancy if people want to use it).

Ultimately, I stopped short of testing the full usage because other things got in the way, but there are some quality of life improvements here too.