Integration of JPSFLib (PSF reference library management)

[ell-bogat]

This PR adds a new module to spaceKLIP for PSF reference library management! Can be imported via:
from spaceKLIP import psflib

The major functionalities are:

• Building a reference database from a directory of JWST files, via psflib.build_refdb()
• Loading a previously generated reference database from a csv, via psflib.load_refdb()
• Downloading the files listed in the reference database directly from MAST, via psflib.download_mast()
• Selecting science and reference filenames for a given target, with the ability to choose references based on parameters like spectral type, filter, etc., via psflib.get_sciref_files(). These filenames can then be passed to the spaceKLIP PSF subtraction routine. More options for useful reference filtering can be added as needed!

[AarynnCarter]

Hi Ell, this is looking great! We'll need to tweak a couple of other files to keep the documentation up to date, but I can handle that in the new year. I left a few comments for you too, but nothing major.