Legacy polarization / energy / plugin helpers under the pxr label.

This package path is deprecated. Prefer the top-level modules (:mod:refloxide.tmm, :mod:refloxide.optics, :mod:refloxide.data, :mod:refloxide.model, :mod:refloxide.objective) and, for the pure-Python TMM, :mod:refloxide.python.tmm. Contents under pxr will relocate out of this namespace in a future release.