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.
You shouldn't have to dilate the old mask again here.
On each iteration you compute a mask and store it in the GammaImages container.

To check whether to terminate all you have to do is get those masks for the old and new container and check if they are the same, no?

(if this isn't clear I can have a closer look at the code)