API Improvements to AbsorberExtractor

[clairekope]

It's slightly confusing and misleading that AbsorberExtractor must initialize using a file. It implies that a new AbsorberExtractor instance must be initialized every time a new ray is to be processed, even though get_absorbers is internally capable of making the supplied AbsorberExtractor object load in new rays.

• I think ray_filename should be removed from the AbsorberExtractor initializer. AbsorberExtractor.load_ray() should be either explicitly called by the user, or called by the get_spice_absorbers and get_spectacle_absorber methods using associated arguments.

Additionally, AbsorberExtractor is initialized with a mixture of arguments that apply to either the SPICE or Spectacle extraction methods.

• These should be moved to their respective methods; e.g., move the verlocity_res argument to get_spice_absorbers and spectacle_res to get_spectacle_absorbers.
• The units_dict and field arguments that get_spice_absorbers currently takes should be made part of the AbsorberExtractor initializer.
• units_dict and field should affect Spectacle data outputs as well. I figure this isn't currently the case because Spectacle was abandoned in favor of SPICE, but if SALSA still wants to support Spectacle I think it should be done.

If/when I start working on these, I'll open a linked WIP PR.