In autoreduce, the last variable settings used in a reduction should count as the 'new defaults'

[Lottie Greenwood]

Currently if a variable is set as 'a' in the default reduce_vars.py script, and then changed to 'b' for x runs, if you then change variables for x+1 runs onwards, it reverts back to 'a' again. Ideally it would instead default to 'b' at this point, because the users will not want to reset all the defaults back to their own configuration just to change it slightly.

Make it so the new default settings for an instrument are whatever the edited runs previously had.

[Stuart Campbell]

This ticket has been transferred to github issue ​12241