Time dependent hamiltonians in 4op 2t correlator

Modified 4op 2t correlator for hamiltonians with time dependence; works with cython or function td format. Added relevant test functions.
This regular expression does not look right.. Consider this example:

    sub("(?<=[^0-9a-zA-Z_])t(?=[^0-9a-zA-Z_])", "t + t0_correlation", "sin(w * t)")

    'sin(w * t + t0_correlation)'

I think at least you need to put t + t0_correlation inside parenthesis.

Also, since t0_correlation is an internal variable, why not just call it _t0 (backspace for private). 

There is no need for the ending \ on like 1040.