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.
I think the intermediate file writing is not tested at the moment ... do this in a temp dir and maybe even add an assert on the outputs, e.g. the sum of the background and mask values or at least that the files are there.