add library for stellar spectra

[zkbt]

The changes include:

• add a library for (hopefully) easy access to PHOENIX model stellar spectra, that automatically downloads preprocessed grids from my website as needed
• add default R options or the ability to cache specific wavelength grids
• add get_phoenix_photons function to grab spectrum for arbitrary temperature, logg, metallicity
• add get_planck_photons function for doing the same with Planck spectrum
• add description of model library stuff to docs