Update cutout overlays

[ddobie]

@Andywang201605 has pointed out that there is now a package that correctly handles plotting sky regions in spherical projections - https://astropy-regions.readthedocs.io/en/stable/index.html.

We could update Source._gen_component_overlays to use astropy-regions.EllipseSkyRegion, which would be much simpler. However this may be a case of "if it ain't broke, don't fix it".

[ajstewart]

Probably be good to revisit this once it has been merged into the astropy core package - as is their intended aim.