Added ability to input time dependent Hamiltonians to the 4op 2t correlator
Necessary in generating test functions (needs mcsolve fixes)
Added test functions for 4op 2t correlator when H is time dependent
Made small changes:
 - regular express
 -added reverse operator argument
 -attempted to change everything to PEP8
 -renamed shift function and a few variables
  -added old-style time dependence
  -shortened test functions
Made small changes:
 - regular express
 -added reverse operator argument
 -attempted to change everything to PEP8
 -renamed shift function and a few variables
  -added old-style time dependence
  -shortened test functions
Corrected typo.  Made H_shifted safer for arbitrary args
Removed old-style time dependence support
Removed old-style time dependence support
Add old-style time dependence support but only for dictionaries
Slightly modified description
- Changed naming conventions for types of td in test_correlation and correlation
- Added protection in hamiltonian shift function if args is None
- Changed "old-style" in mesolve
- Added arbitrary two-correlation to me version of 2op 2t
Fixed error in args is None logic
-removed unnecessary "reverse" argument to 4op 1/2t correlators
-standardized doc strings
-removed unnecessary code for mcsolvers - possibly impement in future
-added error for negative taulist values
-cleanup up a few sections
Reinstated  _correlation_mc_2op_1t
Merge pull request #206 from kafischer/corr-time-dep-H

Time dependent hamiltonians in 4op 2t correlator