Go from Atomistic to UA and from UA to atomistic

[justinGilmer]

Describe the behavior you would like added to mBuild
Allow going between UA/TraPPE representations to Atomistic representations

Describe the solution you'd like
I would like to build up an alkane/alcohol,etc and be able to convert between an atomistic version and to a UA representation.

Would be a nice feature, but is a bit specific.

Describe alternatives you've considered
Sort of an extension of CG-AA, AA-CG

Additional context
N/A

[CalCraven]

#270 and #338 Essentially cover this issue. Combining those issues into this single one, and closing those two, which discuss both directions of this workflow.