Iterative kernel background estimator

Made a new PR as couldn't push to the old one: #173

Still in progress. To do:
 - [ ] Add unit tests
 - [ ] Finish tutorial
 - [ ] Include initial image and mask image additionally in the tutorial
 - [ ] Clean up code formatting
 - [ ] Add docstrings
(Anything else?)

Please comment if you see anything to be done or changed.
No need to document `GammaImages` ... I plan to write a more general class later somewhere else.
Just note here and remove this from the `__all__` list above.
```
TODO: implement a more general images container class that can be re-used in other places as well.
```