Add Polarity branch in MC
Add branches with weights

There are two weights:
1. DsK -> PID calibration sample correction
2. MC -> data correction

Both weights are taken from histograms.
Fix interactive mode for profile plots
Change get_dataset(..) interface

More consistent when passing selection variables.  Also easier to
extend for passing command arguments.
Bug fix: while filling weights, polarities were reversed
Correct for PID and data-MC differences before fit
Bug fix: forgot 2nd coord in 2D hist
Refactor weights from array → separate branches
Update fitting script to use weighted dataset

FIXME: Super-dataset has incorrect weights (RooFit bug)
Workaround: combine dataset with categories by hand

The RooFit::Index(..) command argument seems to ignore weights.
Workaround by adding the categories by hand and then appending the
datasets.