ligo-skymap-plot now uses multi-resolution HEALPix natively

Enhancements to the command-line tool ligo-skymap-plot:

• Read in and reproject sky maps in multi-resolution format. This significantly decreases memory consumption when plotting high-resolution sky maps.

• The algorithm that calculates credible areas printed in the plot legend now matches the algorithm in ligo.skymap.postprocess.crossmatch: it does linear interpolation of area when the credible level falls between pixels.