Fix and enhance functional bag plots.

The median of all series is now extracted by the VTK filter.
In PV, this median is displayed by default along with the outliers,
all the other series are hidden by default.

Change-Id: Ibe09dcd9508097dcb92ae8cf6363e2f0a14d4a99
Merge topic 'FixFunctionalBagPlots' into next

7b7a6621 Fix and enhance functional bag plots.