Airy Thin-film Iridescence Fresnel #317

https://github.com/AcademySoftwareFoundation/MaterialX/blob/main/documents/Specification/MaterialX.PBRSpec.md#thin-film-iridescence

Create a new Fresnel object in moonray/lib/rendering/shading that implements this thin-film model. This replaces the Fresnel term for dielectric, conductor, and Generalized Schlick BSDFs when optional parameters are active, according to the specification.

The MaterialX BRDF development branch for core MoonRay has changes to Fresnel handling & assignment that will need to be extended, but the basic implementation of the Fresnel's math can be implemented ahead of time.