Add snapshot to mbuild Compound backmapping method

[chrisjonesBSU]

Right now, finegrain.py has a method that performs back mapping on an mBuild compound. This PR adds a method that performs the back mapping on a gsd.hoomd.Frame (i.e. Snapshot) and returns an mBuild compound.

Still a couple of things left to do:

• Add doc strings
• Add unit tests
• Figure out if the in-situ energy minimization is working as expected, if not we can take it out
• Add an example to the documentation.

[chrisjonesBSU]

After chatting with @StephMcCallum there was some discussion about how to handle cases where two atoms need to be removed to form monomer-monomer bonds during fine-graining. A common example would be removing the -OH group from something like in polyalanine.

I suggested we keep the behavior as explicit as possible, so that it only ever removes particles you tell it to. To support removing multiple particles when needed, bond_head_index and bond_tail_index need to work if multiple indices are passed in. Then, to make this approach easier, we begin adding a library/database of common chemistries.

If we want to be less strict, we could add a check that if the particle given by one of the bond_x_index parameters is not a hydrogen, then automatically remove only any hydrogens that were also bonded to it.

[chrisjonesBSU]

We need to be careful here. We should iterate through particle.direct_bonds() and make sure we are ony removing ones that are hydrogens.

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