Forcefield-step

* The ligpargen tool was missing a factor of 2 in the dihedral and improper parameters.
* Corrected the search paths for forcefields.
* Improved the documentation.

2024.6.28 -- Added customizable local forcefields and LigParGen interface.
* Added the machinery to handle local forcefield files in either
~/.seamm.d/data/Forcefields (personal) or ~/SEAMM/data/Forcefields (site).
* Added 'ligpargen' command to access custom parameters from the LigParGen service
at Yale University, ading them to the 'ligpargen.frc' personal forcefield, which
is automatically included in 'oplsaa.frc' if it exists.

* The angle parameters for PF6- in the CL&P forcefield only work if the 180º F-P-F
angles are not included in the calculation. Replacing them with an equivalent
periodic SHAPES-like potential almost works; however, since 0º is a valid angle and
there are no 1-3 nonbonds, nothing keeps the F atoms apart. This is solved using a
tabulated potential based on the SHAPES potential but with an added 1-3 repulsion
large enough that the gradient is always pusing small angles apart, but not large
enough to affect the minimum at 90º.

2023.9.14 -- Fixed errors! And added C2mim to test.
* The units of the torsions were incorrect in the last implementation.
* Added parameters for 1-alkyl-3-methylimidazolium cations from JCP 108, 2038 (2004)
* Tested much more thoroughly.

* Parameters for TFSI - bis[(trifluoromethyl)sulfonyl]imide

* cyclopropane -CH2-, -CHR-, and -CR2-
* hexafluorobenzene
* difluorobenzene
* bromobenzene
* iodobenzene
* thiophenol
* alkyl nitriles
* nitroalkanes
* nitrobenzene
* methylene in phenylacetonitrile
* corrections to methylene nitrile anion

* The Lennard-Jones repulsive term added to the F-P-F angle was too weak, allowing the
structure to get trapped in a symmetric state with ~40º angles.