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.
This is quite a bit of text and not all is 100% correct.
I'd prefer a very brief description here (1 sentence) and a TODO that we should add a link to the proceeding you're writing ... that's the place where the method should be described in detail.