Unit test paths appear to break on Windows

[Sujay-Shankar]

local_files = glob.glob("**/SDCH*.spec_a0v.fits", recursive=True)
file = local_files[0]

^ throws an index error

[gully]

We should use either pathlib or os.path... to make these paths platform independent.

I suspect the same problem may true in other places, where we assume Unix-like paths... Let's scan through both muler and gollum for this assumption. Also, I wonder if it's worth setting up a Continuous Integration GitHub Actions matrix on Windows...