Conventions-free merging weight taken into account
See discussion concerning "Merging:includeWeightInXsection" at https://indico.cern.ch/event/958976/
Added small improvements in view of Pythia8.3 compatibility.
Added also checks needed by https://its.cern.ch/jira/browse/AGENE-1915 (this ended up in a fresh new python module Pythia8Util, plus its base fragment Pythia8_Pythia8Util.py)
Edited by Giancarlo Panizzo