Forcefield-step

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 pushing small angles apart, but not large enough to affect the minimum at 90º.

- Fixed significant problems in yesterday's release, mainly the wrong units for torsions.
- Added CL&P parameters for 1-alkyl-3-methyl imidazolium cations

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

Added typing for the following functional groups, which were missed in the first implementation.
* cyclopropane -CH2-, -CHR-, and -CR2-
* hexafluorobenzene
* difluorobenzene
* bromobenzene
* iodobenzene
* thiophenol
* alkyl nitriles
* nitroalkanes
* nitrobenzene
* methylene in phenylacetonitrile
* corrections to methylene nitrile anion

The typing for this and few other atom types were forgotten in the first round. More will be added soon for e.g. perfluorobenzene

The Lennard-Jones repulsion added to the angle was not quite sufficient to stop it having a metastable structure with an FPF angle of ~40º.