SIMD acceleration for RB dihedrals

RB dihedrals now use SIMD acceleration analogous to proper dihedrals
when no energy and virial is required. This also significantly
improves load balancing (issues) for systems with proper+RB dihedrals.
Refs #1598.

Change-Id: I07000125d19db45fc35e1a0c28149c8a19443680