Consistent plot formatting

I noticed that the plotting in the gw result module wasn't being run in the CI which put the coverage pretty low.

This MR adds some testing to this module.

I had to change an option in the skymap plot. It looks like the API for the ligo.skymap.kde.Clustered2DSkyKDE changed at ligo.skymap@6ce94db4. Are there complaints with just changing this? ligo.skymap doesn't seem to be in the requirements anywhere.

Update: I set it so that the core plots now use the same basic formatting as the gw plots. This is basically just setting the font to Computer Modern Roman (the default latex font) and attempting to save all plots with usetex on.