Updated doc strings & removed mc version of 2op1t

Also:
-changed some instances tlist to taulist for clarification
-changed normalized correlation functions to steady state only

I made a noticed a few mistakes in the docstrings and now I think all the inconsistencies are fixed.

Re the mc correlator:

It looks to me like the quantum regression theorem was attempted to be applied here but the initial condition requires a density matrix. However, the MC method only takes an initial condition as a state so this would not work in general. I suppose in principle there could be special cases that work, but I think the MC correlator needs a different technique.
should there be a new public function "correlation_2t" or just "correlation_3op_1/2t"?