Seamm-ff-util

* The code allowed the torsion around a 3-membered ring which had the same atom at
each end of the torsion. This is not a valid torsion, and the code now checks for
it and removes it.

* Added support for local forcefields files which can either be used directly
or included by existing files.
* Added URI handler to support local files
* Added support for BibTex references in forcefield files, and automatically adding
citations to the Reference Handler.
* Add 'fragments' section to forcefields for atom-typing via a fragment or entire
molecule. This supports using LigParGen for OPLS-AA forcefields.

* Also improved unit handling across all terms in forcefields.

* Added the molecule number for each atom for when using LAMMPS

* Added handling of the OPLS-AA forcefield
* Moved documentation to new MolSSI theme and diátaxis layout
* Cleaned up internal dependencies and workflows for GitHub