External/Pythia8 and Generators/Pythia8_i were working with the Pythia 8.3 series with two exceptions, which this MR is fixing:
soft qcd processes: Pythia8.3 needs, on top of the share/Pythia8/xmldoc subfolder, also share/Pythia8/pdfdata, which is now properly copied into the installArea by External/Pythia8/CMakeLists.txt
UserHooks: Pythia8.3 changed the internal user hooks pointer from UserHooks* to shared_ptr<UserHooks*>. This must be managed with some care when trying to support both Pythia8.2 and Pythia8.3, which is now done by our interface Pythia8_i
Simple technical tests are passed. Now needs physics validation, out of the scope of this MR.