Iterative kernel background estimator

Made a new PR as couldn't push to the old one: #173

Still in progress. To do:
 - [ ] Add unit tests
 - [ ] Finish tutorial
 - [ ] Include initial image and mask image additionally in the tutorial
 - [ ] Clean up code formatting
 - [ ] Add docstrings
(Anything else?)

Please comment if you see anything to be done or changed.
Use relative imports for Gammapy, move Astropy imports to the top.