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.
We want to use `Quantity` objects as much as possible.
These radii should be `Angle` objects (Angle is a Quantity sub-class).
Please adapt the background estimator API for this to work nicely.