Adding JO example in Pythai8 for the new UserHook

I Added a new JO example for Pythia8 using the new UserHook mergingDJRs

Generators/Pythia8_i/share/example/mergingDJRs_example.py

+# Check Pythia8 README for more details and the full steps on how to generate the lhe file.
+# In this example, we considered the production of Z+2jets where Z decays to e+e-
+
+include("Pythia8_i/Pythia8_A14_NNPDF23LO_EvtGen_Common.py")
+
+#### Shower
+evgenConfig.description = 'Example UserHook for computing CKKWL merging DJRs'
+evgenConfig.keywords+=['Z','jets']
+evgenConfig.generators += ['MadGraph', 'EvtGen', 'Pythia8']
+
+include("Pythia8_i/Pythia8_LHEF.py")
+
+PYTHIA8_nJetMax=2
+PYTHIA8_Process='pp>e-e+'
+PYTHIA8_Dparameter=0.4
+PYTHIA8_TMS=20.0
+PYTHIA8_nQuarksMerge=4
+
+# Include to activate the CKKWL merging
+include("Pythia8_i/Pythia8_CKKWL_kTMerge.py")
+
+# Include to compute the DJRs and save them in ROOT file
+include("Pythia8_i/Pythia8_mergingDJRs.py")
+