Add jwst_download.py script

[bhilbert4]

This PR reorganizes things under the hood, in a way that should be invisible to users. Rather than having the jwst_download.py module installed as a script, this PR creates a scripts subdirectory and creates a jwst_download.py script. The script simply imports the jwst_download.py module and runs it. The script essentially contains what used to be in the if name==__main__ section of the jwst_download.py module.

I've checked that when jwst_mast_query is installed, jwst_download.py is available as a script from any location, and that it queries MAST and downloads files successfully.

[bhilbert4]

@arminrest I think this is what you are looking for?
Literally all I did was take the if name=='main' part of jwst_download.py and put it in a new script file (also called jwst_download.py) in a scripts subdirectory. The jwst_download.py module was previously set up to be installed as a script. I think it may not have worked, but only because one of the import statements was written incorrectly. That's fixed in here as well. So in theory I think we could fix that import and probably be done. Or we could keep this separate script file as well.

[bhilbert4]

Talked about this offline with Armin, and we decided it's ok to merge.