Add support for writing rigid bodies when converting to HOOMD formats.

[chrisjonesBSU]

rigid IDs are ints assigned to particles/atoms that signal which rigid body they belong to which is needed when recording rigid body information in HOOMD Frames and/or GSD files. The mBuild hoomd writers handled this, but we still need to implement them here.

In mBuild, rigid_id is a property at the particle level, so we can treat it the same here, at the site level.

This is a draft for now. Remaining TODOs are:

• handle cases where some molecules are rigid and others aren't
• throw an error if rigid particle type is found in the topology already

[CalCraven]

A remaining question to figure out: What if only some molecules are rigid in a hoomd simulation? How does the list of rigid body indices change?
https://hoomd-blue.readthedocs.io/en/v3.6.0/tutorial/06-Modelling-Rigid-Bodies/00-index.html

[CalCraven]

Was playing around with the rigid bodies in HOOMD. Got a working simulation with some rigid particles and some non-rigid particles. Looks like the body argument, shown now in line, get's a tag of -1 if the particle is not rigid. The rigid particles get the rigid ID for that particle, and the rigid COM particle, named "dimer" here, also get's that same ID.

[CalCraven]

Mostly a bunch of doc string suggestions.

[CalCraven]

We should add a test also checking for the error that comes up when an mBuild object is generated with rigid molecules at the back of the compound list, where the snapshot rigid stuff will fail. Maybe we can write a descriptive error for that case to catch.

[CalCraven]

This LGTM! Should be ready to merge

[CalCraven]

Let's be sure to raise this issue, maybe we could make a new GMSO issue for breaking changes that we should start collating.

[chrisjonesBSU]

I'm just going to add a check for the hierarchy ordering of rigid and non-rigid, then it should be good to go.