Backtransform derivatives on symmetry particles

[benmwebb]

core::TransformationSymmetry and core::TransformationAndReflectionSymmetry make sure that a particle P' is kept with its coordinates derived from a reference particle P (by transformation and optionally also reflection). However, any forces on P' are not mapped back to P. This would break MD, CG etc. on such a system. Investigate methods to perform this backtransform and provide an example.