remove Gala dependency

[abostroem]

How could the content be improved?

Add gd1.py (https://github.com/adrn/gala/blob/main/gala/coordinates/gd1.py) and reflex.py (https://github.com/adrn/gala/blob/main/gala/coordinates/reflex.py) to student download to remove gala dependency

Gala tries to compile somethings in C which breaks on windows. This has been a pain point for the last few workshops and we were not able to get around it this workshop. We don't need any of this optimization for our workshop and the files we do use are stable and pure astropy. I suggest that we download these two files and include them in the student download. Then we can remove the gala dependency from the workshop setup.