Refactoring of add_model and support for Sonora models

[tomasstolker]

• Refactoring of Database.add_model, making the code fully generic. This also allows users to add their own model spectra.
• Added Database.available_models to print information about all available models that can be added to the database. It fetches model details from data/model_data.json.
• Support for the Sonora Bobcat and Sonora Cholla model spectra.
• Fixed an issue in update_objectbox with the photometric error inflation that caused an error.
• Some improvements to documentation and docstrings.

[tomasstolker]

The Bobcat Sonora spectra are added with the label "bobcat-sonora" or "bobcat-sonora-co". For the latter, log(g)=5 and [Fe/H]=0, but C/O is a free parameter. The Bobcat Cholla spectra are added with the label "bobcat-cholla" and have log(Kzz) as free parameter to account for disequilibrium chemistry.