Refinements to polymer_examples-derived code

[timbernat]

Certain polymerist modules are essentially clones of scripts found in openff/polymer_examples, written by Connor Davel. These scripts would be greatly improved by being made more consistent with the rest of polymerist, in terms of functionality, consistency, and readability. The most obvious of such changes are:

• openfftools.partition : would be desirable to separate single-mol partitioning from whole topology partitioning
• openfftools.partition : would like to have a separate method to merely check where a partition already exists (rather than just attempting to repartition always)
• openfftools.partition : would like to migrate graph traversal to other, standardized library (perhaps rdutils.rdgraphs)
• monomers.conversion : finish absorbing this module into monomers.specification
  - [ ] monomers..specification : implement support for SDF, RDKit Mol, and SMILES file I/O. Implement limit on replacements for similar environments (via recursive SMARTS)