Test case for Python 3 sky map generator
This adds a test case for the Python 3 sky map generator
It will need to be run with an up-to-date version of the dependencies, so the Python 3 dependencies are available. The test case should also be run after load_spiir, so it knows what branch you're testing.
It follows the existing unit test design, and has two parts: a test script 'run_gw170817_skymap.sh' and a 'skymap_comparison' function within create.py.
All the test data are stored in the 'fits_skymap' directory within unit_testing, using LFS.