Rewrite all-sky plotting to use wcsaxes and reproject
This is a modernization rewrite of the all-sky plotting codes in LALInference, using
astropy.visualization.wcsaxes for astronomical projections and
reproject for rasterizing HEALPix images.
This is a cleaned-up version of the plotting codes that I used to generate sky map figures for the GW170817 papers.
Note: I'm expecting some initial build failures because I am probably using a lot of bleeding-edge features from Astropy, not to mention adding reproject as a new dependency.