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.
Here and above, on line 1036 and forward, you assume that args is a list and dictionary, respectively, but args is really supposed to be a arbitrary something that is passed to the function td-function. Instead of doing it this way, how about just setting

    _args = [args, 0]

and leave the user defined args untouched, and in for example line 1038 you could do

     H_shifted = lambda t, args_i: H(t+args_i[1], args_i[0])

and on line 1054

    fn = lambda t, args_i: H[i][1](t+args_i[1], args_i[0])

Since we haven't really specified that args must be a list or dictionary for the two formats, I think this would be a bit safer.