Implementing Faradaty Rotation Material

[bryancole]

I want to model an optical isolator. This requires modelling Faraday rotation. This is probably easier than might be expected. I'm planning on making a FaradayRotationMaterial cmaterial class which works as a wrapper class around one of the other more conventional dielectric materials. Faraday rotation is sufficiently decoupled from the dielectric permitivity that they can operate independently in most physically sane cases.