Add `rigid_id` field to `Atom` and handle 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:

• [ ] Parse rigid_id in to_mbuild
• [ ] Parse rigid_id in external.convert_hoomd.to_gsd_snapshot and to_hoomd_snapshot
• [ ] Add tests