Extracting monomer from typed mol2 file

Currently, the code parameterizes any monomer (yay!) and renames the atoms to match the atom typing done by espaloma (also yay!). However, when you load an mBuild compound using the typed file it's difficult to extract the monomer from the mBuild compound (it includes both the monomer and dimer) to be used for building a polymer.

We could delete the dimer out of the mbuild compound, but the issue it determining which indices belong to the dimer and how to delete them without doing it by hand.

This is in the file esp_xml.ipynb in the combo branch:
[https://github.com/cmelab/forcefields/tree/combo](url)