How to handle virtual_sites in to_mbuild?

[CalCraven]

Describe the behavior you would like added to GMSO
If you have a Topology with virtual_sites, should those be optionally passable as sites to mBuild, or even Parmed? This is a place to have that discussion.

See #890 for examples of virtual_sites usage in Topology and tests.

Describe the solution you'd like
Open for discussion.

Describe alternatives you've considered
Open for discussion.

Additional context
N/A

[chrisjonesBSU]

I think we could add a parameter (something like write_virtual_sites=True/False) to to_mbuild and to_parmed, but also the file format writers (xyz, gro, json, etc..)?