Remove SkyMask (merge with SkyImage) #966
This pull request removes SkyMask, instead using SkyImage everywhere.
The reason is that we're now getting more image and cube classes and I think that a separate hierarchy of corresponding mask classes doesn't make sense. Generally I'm a fan of having separate classes for separate things, but in this case I don't see the advantage of splitting masks out. At the moment there's one useful property "distance_image" which I'm moving from SkyImage to SkyMask, and even if there are a few more, I think it'll be OK to just check that
Maybe add a helper method
Tests don't pass here at the moment. I'll make a PR to fix this issue with