Consistency checks

Here we compare the behaviour of GILDAS and other astronomical software regarding GLS and SFL projections. Comparisons are done using the GREG CONVERT command, astropy.wcs Python module version 5.2.2, and the FITS graphical viewer ds9 version 8.2.1. The test data is created as a large scale map, with all values blanked except 3 single pixels for easy identification: $(3600,-1800), (3600,+1800)$ and $(-200,0)$ arcseconds relative offsets on the projection plane, and checking what the various software return as corresponding absolute celestial position on sky. Note that the ambiguity is about 1 pixel (5 arcsec in the test data) which is good enough to detect problems on such large maps.