Updated doc strings & removed mc version of 2op1t
-changed some instances tlist to taulist for clarification
-changed normalized correlation functions to steady state only
fixed typo
corrected another docstring
Combined spectrum_ss and spectrum_pi functions into spectrum function with *solver* input string
Added functions to computing two-time correlations with mcsolve
- needs lots of organizing (next step) but they all work now
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.
Reinstated API:
-new spectrum command (specum_pi/ss now legacy)
-added warnings for legacy
-all correlation functions now call _correlation_2t
-added new test functions for new functions over legacy functions
-refined doc strings

todo:
-add mcsolve 'steady state'
-suppress mcsolve output
-something is broken about the test for the mc correlator - need to suppress mcsolve output so I can see what this is...
Merge branch 'master' into correlation_fixes_minor
suppressed mcsolve output in correlator & fixed mc correlation test function
Removed unnecessary lines
Correction for tlist=None
fixed typos
Another typo
Fixed typo yet again
Removed unnecessary future warning
Removed flag
Updated doc strings
Added mc_corr_eps to doc strings
Removed all work on mc correlator steady state - decided this was currently too much work to implement well.
Merge branch 'master' of https://github.com/jrjohansson/qutip