Add support for time-dependent Hamiltonians

[homocomputeris]

Follows from #380.

If split and 2nd-order time-dependent problems can be solved, then problems with corresponding time-dependent Hamiltoninans can be solved too.

Some useful references:

• The general approach is described in Time-Average on the Numerical Integration of Nonautonomous Differential Equations (preprint).
• An example of Splitting methods for non-autonomous linear systems

[ChrisRackauckas]

This could make a good GSoC project.

[joeyli99]

I meet a same question. I was wondering if there is any progress?

[ChrisRackauckas]

For second order methods, our current state is the form

http://docs.juliadiffeq.org/latest/types/dynamical_types.html http://docs.juliadiffeq.org/latest/solvers/dynamical_solve.html

The solvers technically let you give it a t argument, so you can define a partitioned ODE from a time-dependent Hamtiltonian. We also have https://github.com/JuliaDiffEq/ExponentialUtilities.jl for doing fast matrix exponentials. So we are just about there.