Iterative kernel background estimator

Made a new PR as couldn't push to the old one: #173

Still in progress. To do:
 - [x] Add unit tests
 - [x] Finish tutorial
 - [x] Include initial image and mask image additionally in the tutorial
 - [x] Clean up code formatting
 - [x] Add docstrings
(Anything else?)

Please comment if you see anything to be done or changed.
Sure, - I think the problem is that it just doesn't work well on small low resolution images like the test ones we have in FermiGalacticCenter. I could alternatively put a higher res counts image into gammapy-extra which is downloaded, then the images should work out OK here? I'm not sure how you rate the pros & cons of calling a download of a larger file compared to putting the link to the notebook example (the API has changed quite a bit since images in the notebook were produced for instance)