Remove SkyImage and SkyCube

[cdeil]

This pull request removes gammapy.image.SkyImage and gammapy.cube.SkyCube and thus finishes the transition in Gammapy to the new gammapy.maps.

It is the culmination of 1-2 years of work, mostly by @woodmd to design and implement gammapy.maps, and then especially in the past months our work to develop analysis code based on gammapy.maps, and a very concerted and efficient transition and cleanup in the past month.

There will be significant more work in the coming days to update the gammapy.image docs, several tutorial notebooks, and to improve the new gammapy.maps based analysis functions and classes.

Users that want to update from Gammapy v0.7 to v0.8 will all have to learn gammapy.maps and update their scripts. We realise this is painful. To help, we have prepared an intro to gammapy.maps tutorial and will update all high-level docs in Gammapy in the next days. We're also happy to answer any questions on Slack or the mailing list and help update scripts to the new code.

There was the possibility of keeping the old for a few more months, and to have both the old and new in parallel. This would have been a large drag for Gammapy developers, and I also think it would have been a nightmare for users, to see a wild mix of code and docs using SkyImage/SkyCube or gammapy.maps. The Gammapy userbase will grow significantly in the coming year, and for new users having the transition done is gold. For existing users: consider sticking with Gammapy v0.7 for existing working scripts that use SkyImage / SkyCube, and for new projects or projects that just started, update to gammapy.maps. For the vast majority of code, it's really mostly equivalent and you "just" have to update one line at a time until it runs again and you get the same output as before. Ask for help if you're stuck or don't know how to do it!