Fix dropping bins with `b` step when AutomaticDropBins is used

Mohamed Alyrequested to merge
patch-blinding-cleanup into master

Executive summary of changes:

Currently if AutomaticDropBins is used the code will look for region->fBlindedBins list to use for creating the _dropBin histograms. fBlindedBins is currently only assigned a value right before plotting, meaning that the code will never find automatically dropped bins.

This is not a problem for manually dropped bins, since there are assigned at config reading stage.

This MR assigns to fBlindedBins for all regions once before the b step is ran, then again before plotting in case histograms have changed.

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