efficiency plots with reference

@dovombru, this MR is to make the efficiency plot with the reference, based on lhcb-head.2037.

Please let me know if the plotting is fine. You can check the example here: https://lblhcbpr.cern.ch/#/app/rootfileviewer?apps=5&withNightly=false&checkSpecific=false&nightlyVersionNumber=10&jobs=62606

To update the reference, one has to update the corresponding environment variable in jenkins node where the test is running. How often do you think the reference will be updated?

The drawback of this solution is that in the output ROOT file there are also TGraphs appearing in the list of objects in the file. But on the other hand, thanks to that the process of updating the reference file is simplified.