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.
Personally I prefer not to have abbreviations in function names (because abbreviations can be ambiguous... was it, fn, fcn, func, ... ?), so I prefer coherence_function for a new function name rather than coherence_fn. A strong case can be made for both conventions, but is is preferable to be consistent within one project, and in qutip we usually spell out names when possible, so I'd prefer if we stick to that convention.

Also coherence_function_g1 still needs to be defined.