GWSkyNet handles flat-resolution sky maps, poses risk of memory exhaustion

GWSkyNet's model evaluation code contains code that generates 2D and volumetric plots in-memory using code adapted from old versions of ligo-skymap-plot and ligo-skymap-plot-volume respectively, in: src/GWSkyNet/GWSkyNet.py. This defeats the purpose of the code changes to use multi-resolution HEALPix maps for plotting (see lscsoft/ligo.skymap!339 (merged), lscsoft/ligo.skymap!408 (merged)). It poses a risk of memory exhaustion.

Because GWSkyNet contains copies of parts of ligo-skymap-plot-volume, it references the internal function volume_render. The signature of that function changed in a way that cannot be separated from the changes that I made in order to switch to using multi-order sky maps when making plots (see lscsoft/ligo.skymap!408 (merged)). As a result, GWSkyNet has a dependency on ligo.skymap <= 2.0.1.