Time dependent hamiltonians in 4op 2t correlator

Modified 4op 2t correlator for hamiltonians with time dependence; works with cython or function td format. Added relevant test functions.
Yes, that makes sense, I was actually imagining after agreeing on good form/style for this I would add functionality for time-dependent collapse operators then also add these to the other non-steady state correlator - if you thought that made sense.