Load image data when button is clicked

Closes #13
Add widget displaying a calibration step

By which I mean something like subtract_bias or gain_correct
Add action attribute for controller to hook into
Add display method to ToggleContainerWidget
Add reduction steps to ReductionSettings
Example dark reduction button
Draft of new overscan/slice widget
Switch to sensible guy for overscan/trim

Closes #20
Progress on #19 #15
Add really basic documentation