gromacs: avoid dihedrals close to 180/-180 periodicity

Due to the way gromacs calculations dihedrals we end up with issues when crossing from 180 to -180 on a dihedral. To avoid this we should just make sure that angles that are within a given cutoff of that boundary are given really high variances (so that they won't get picked).

To do - this is documented in one of Vytas' recent papers on NEQ absolute binding free energy calculations.