3D solver

[twhughes]

Is your feature request related to a problem? Please describe.
Right now we do 2D only, could try doing 3D problems.

Describe the solution you'd like
Should be very similar to before, except we will check if its a 3D simulation, and then use an iterative solver.

Describe alternatives you've considered
Is it worth doing this at all? 3D adjoint FDFD is very computationally expensive and FDTD would probably be better.