Add check to remove zero Charmm dihedrals

Proper torsions where the force constant is zero
in both A and B states are now removed. We also
check for other angle, torsion, and restraint
functional types, and if all parameters are zero
for these the interaction is not added. This will
not change any results, but increase performance
slightly by not calculating unnecessary interactions.
Fixes #810.

Change-Id: I37ecd06d0641008593edab29e5b08433bde7b6cc
Merge "Add check to remove zero Charmm dihedrals" into release-4-6