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 docstring needs a `Parameters` and `Returns` section where you document the inputs and outputs.

`n_iterations` should be renamed to `max_iterations`.

And can you rename `auto_termination` to `terminate_when_mask_stable`?
Or remove this option altogether, as I said yesterday there is no use case for iterating when the result doesn't change any more.