Analytical approximation for standard ray-tracing textures

[phoebe-p]

For common surfaces such as (obviously) a planar surface, regular pyramids, or V-grooves, the possible ray paths are limited, specifically at normal incidence but even at small non-normal angles of incidence. Using TMM calculations with local angles of incidences for the different faces in the surface is much faster than ray-tracing and for many cases will be just as accurate, or give a good approximation for e.g. optimising anti-reflection coatings. This could be implemented for internal surfaces as well, but would be most relevant for the front surface, since there is usually a range of wavelengths where light does not reach the back surface at all.

To do:

• [ ] Add functions for (semi-)analytical ray-tracing
• [ ] Add relevant user options for controlling this behaviour
• [ ] Add tests
• [ ] Add documentation, specifying for which interfaces this method has been checked and is accurate (depending on angle of incidence!)
• [ ] Quantify when this is a good substitute for Monte-Carlo ray-tracing simulations