Add to_rdkit functionality

[chrisiacovella]

Describe the behavior you would like added to mBuild
Add in a function to convert a compound to rdkit. We already have support to go from_rdkit (as that is what drives loading from smiles). This is particularly useful for displaying a chemdraw style rendering of a compound.

To have this work better, we'll want to store bond order as well. This can be easily added to the bond graph as a data description for an edge in networkx.