adding new UserHook to compute CKKWL merging DJRs for Pythia8
Added a new UserHook and a common JO for Pythia8. The UserHook is responsible for computing the merging scales called the DJRs and storing them as ROOT histograms. The code will help ATLAS users do deduce whether the merging scheme is working properly and if the merging scale aligns correctly with their study.