Polarization independence

[twhughes]

Is your feature request related to a problem? Please describe.
Right now, we specify the polarization of the problem, and then all fields are defined in that polarization.

Describe the solution you'd like
Would be great if simulation objects were defined as containing both Ez and Hz polarizations. Then, the current sources can be used to specify which fields are non-zero. For example:

S = simulation(...)          # new simulation independent of polarization
S.src['Jz'] = new_src1     # now Ez, Hx, and Hy may be solved for.
S.src['Mz'] = new_src2     # now Hz, Ex, and Ey may be non-zero as well.

This will allow us to do optimization for problems that have components from both polarizations (ie polarization splitter).

[shoyer]

An alternative way to set this up would be to allow multiple simulations to be used in a single optimization. That might make for a slightly cleaner separation.