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.
Import only from `gammapy.image`, not from `gammapy.image.utils`.
I've written up why [here](https://gammapy.readthedocs.org/en/latest/development/index.html#where-should-i-import-from)