Update stellar model libraries for chromatic.

[zkbt]

This draft starts to improve the stellar spectrum models.

• The first tiny fix is a patch for downloads not working with the old Husser et al. (2013) PHOENIX models that were pre-made and stored at https://casa.colorado.edu/~bertathompson/chromatic/. This is probably (?) a problem with the CASA server, but as a slightly risky fix we added allow_insecure=True to the astropy downloader.

[zkbt]

@Pat-Wachiraphan , this should fix the immediate download problem in #243 , but it doesn't yet expand the libraries we're using like in #241 . I'll keep working on it when I can!