OBE

[hartytp]

Add an OBE solver.

Current plan is to use QuantumOptics.jl as a backend and just make a simple wrapper. The one slight concern I have with that is how darn slow julia is for startup - but it still feels better than the options like (a) Qutip or (b) role our own efficient OBE solver in pure python probably with a lot of numba (which is basically writing c but pretending it's python).

• the interface should allow the user to pass in additional Hamiltonian terms / superoperators
• do we want to have a way for the user to expand the state space (e.g. to include motion of atoms)?