An easier path to pre-caching the PHOENIX grids

[gully]

@SujayShankarUT added download=True to PHOENIXGrid, which allows easy pre-caching of the grids, automatically by astropy! We had previously talked about adding a way to precache the model grids to lessen the manual setup burden for new users. This simple flag makes it really simple now! 🥳

There is still some more we may want to do, including:

• preserve the PHOENIX directory structure (rather than obfuscated astropy blob names)
• Update guidance to new users
• etc...