A prototype of keyword parameters

Look at the harmonic_bond for gromacs (And writing for lammps).

This could be made much more compact and clean with a rewrite of forcedata.
Now have a way to have unit arrays for lammps, even though
they get set later based on the requested unitset.
Working to new keyword approach.
Updated a lot of changes to use kwforces.
Keyword parameters now supported for gromacs (one bug)

One error in dihedral parsing (I think writing) needs to be tracked down.
Easiest way it to compare what is in develop.
Fixes for most of dihedral bugs.

Still one more with multiple dihedral type 9.
Merge branch kwforce into develop