Rewrote API and added new functionality:
- removed all correlation functions and rewrote one as correlation_2t. It computes all possible forms of 2-time correlations (includes the previous 2op and reverse functionality) from the 3operator version. 2op correlations are computed by setting one of the three to the identity.
- the new function works for me, es, and mc solvers. both me and mc work for time-dependent hamiltonians
- still need to implement mc "steady state" (see doc string for what I plan to do)
- the old api can be added back if necessary BUT 2op_1t is super misleading IMO so I think it should not be added back - I got quite confused over it.  It still calculates a two time correlation just along one time axis, not a one time correlation as its name implies. I also really like not having this as it emphasizes the two-time nature, that a second time does always need to be specified/implied.