Factor out combination choice into separate widget
Use DarkSubtractWidget

Closes #49
Remove unneeded display method

Closes #50
Move is_sane method into ToggleGoWIdget

Closes #51 and reduces duplication
Move destination and apply_to properties into new class

Closes #52
Moves group_by settings from CombinerWidget into its own widget

Closes #53
Move group_by logic out of CombinerWidget into GroupByWidget

Closes #54
Add scaling option to CombinerWidget

Closes #55
Add flat correct widget

Closes #56
Hack to delete 'subdark' keyword if present

It leads to a io.fits error that I do not understand
Refactor CalibrationStepWidget to allow for matching on master name

and also modify ReductionSettings to use this functionality
Modify notebook to include flat reduction and light reduction