Add subset of USNO catalog as astrometric_refcat 2
Replaces !2 (closed), which does not use git lfs, this does.
Copied from /Software/users/astro-wise/catalog/USNO-A2 at Kapteyn. Contains the necessary data to run astro/test/processing/LDAC_test.py
env astrometric_refcat=/unittestdata/astro/catalog/USNO-A2/ python -m pytest astro/test/processing/LDAC_test.py