Added initial module docstring.
Converted argument types from ndarray to double memory views.
Improved various glass features.
Added subsampling of the Sellmeier equation to produce more accurate refractive index samples.
Added caching to Sellmeier spectral function.
Added external_index to Glass to allow users to override the external refractive index used in the refraction/reflection calculations. By default the external refractive index is set to 1.0.