Optionally write GROMACS topologies as multiple .itp files

[mattwthompson]

Description

Resolves #95

This is opt-in with a monolithic=False argument wired through various methods.

Currently, this splits out monolithic topology files into

• A single f"{prefix}_atomtypes.itp containing non-bonded parameters
  • Currently this is just [ atomtypes ] and I don't see anything else in $CONDA_PREFIX/share/gromacs/top/*.ff/ffnonbonded.itp that indicates other directives conventionally go here
• A f"{prefix}_{molecule_name}.itp file for each unique molecule type, containing each molecule type's atom types, charges, valence parameters, constraints, virtual sites, etc.
  • The molecule name is meant to be a canonicalized version of the Molecule.name attribute

I don't think this actually reduces the net file size on disk, barring sketchy force field re-use, but it does split it across several files which might make things more human-readable.

I doubt it's relevant to current OpenFE work but cc @hannahbaumann

Checklist

• Add tests
• Lint
• Update docstrings

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 93.69%. Comparing base (fcf4399) to head (a80a54d).

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