Improve OpenFF interface

[andrewtarzia]

Notes from this presentation: https://www.youtube.com/watch?v=uvhZBpdDjoM

• Solvation - use PDBFixer from OpenMM (addSolvent)
• To soften forcefields for the "constrained" process use:

softened_ff = ForceField("")
for parameter in softened_ff.getparameter_handler("vdW"):
    parameter.rmin_half *= 0.5

• We need to check that the partial charge is performed on the cartesian coodinates because that is the main issue with undefined stereo flag In OpenMM/OpenFF optimisers, handle define_stereo flag. #182