Microfacet roughness missing

The exporter doesn't export the roughness of Microfacet BSDFs. While there is no "correct" relation from alpha to roughness the pbrt code gives us one: https://github.com/mmp/pbrt-v3/blob/13d871faae88233b327d04cda24022b8bb0093ee/src/core/microfacet.h#L83

I'm not sure if it's worth implementing this or keeping it as the nori_float_alpha custom property.