Interface roughness layers

It would be nice to have interface roughness layers at the top and bottom of a certain layer. It is generally accepted to fit the roughness of a thin film with an EMA at the top and bottom, using the index of refraction of the same layer of interest as `nhost` and something else as the `ninc`.

Start by inspecting the repository's existing layer and EMA APIs, since the issue names no files or tests. Determine how top and bottom interface roughness should be represented and how `nhost` and `ninc` are selected; done means the behavior is specified and covered for both interfaces.

Written by the indexing model from the issue text.