Add support for energy evaluation to update.MuVT()

enables, among other things, Gibbs Ensemble with patches. Should be merged after patch_union.

• add PatchEnergyJITUnion, compiles

• added jit.patch.user_union()

  it runs .. slowly ..

• optimize energy calculation

  only take into account neighbors within r_cut

• be on the safe side with r_cut initialization

• fix cutoff

• add patch support to update.muvt()