Initial version that codes part of a reduction
Convert action to a method
Add functioning CombinerWidget
Add class for performing bias subtraction
Update notebook to reflect updated calibration flow.
Rename keys property

Name conflicts with the keys property of an IPython widget
Rename ReductionSettings to ReductionWidget

It is no longer just settings anyway...
Change name of image_collection argument

Preparation for adding a second collection representing the reduced images
Implement DarkReductionWidget and update bias widget
Initial implementation of CombinerWidget actions
Update notebook to match changes in widget APIs