General surface interaction

[qualand]

Legacy version of SolTrace only supports reflection and refraction surfaces. These surface types enable reflectivity-only or transmissivity-only interactions, respectively. While users can create a "layered" approach, where two surfaces (one reflection and refraction) are places close together to model a "real" surface.

TODO: Create a general surface type where users can specify reflectivity, transmissivity, and absorptivity.