Update Pythia8_i.cxx, see https://its.cern.ch/jira/browse/AGENE-1866
For shower weight variations, Pythia8 from 8.235 to 8.243 silently switched from the concept of "weight" to that of "group weight", documenting it only from 8.244 and making it truly working. The Pythia8_i interface, already updated to manage the intermediate 8.235-8.243 versions, was not yet instructed not to fix 8.244, which finally works without workarounds.