FlavorTagDiscriminants: add scalar track pt sum to output
This MR adds an additional output variable as a decorator to the BTagging object.
The output variable is defined as the scalar sum of the track pt of the tracks associated to the jet. It can be used e.g. for defining relative pt fraction of tracks w.r.t. this quantity or for constructing an observable which can be used for flavour tagging algorithm training (see attached plot).
