TResAnalysisPlot feature required to allow stacking the histograms

[jgalan]

There is a ROOT class named THStack. That class could be used to add the histograms in a given <plot and if an option stack="on" is enabled we then present the accumulated histograms one on top of each other.

https://root.cern.ch/doc/master/classTHStack.html

This is useful to understand the different contributions to the background and also get the idea of total contribution at once in a single histogram.