Merge pull request #5551 from bendavid/pythia8200pre2interface

Pythia8.200pre2 interface changes
properly initialize event attributes in pythia8 lhe interface
update key configuration snippets for pythia 8.2 and create organized fragments for common pythia8 settings
improve comment in config file
proper accounting for merging weights from CKKWL, UMEPS, UNLOPS
Merge pull request #5602 from bendavid/pythia8_200pre2_72x

Pythia 8.200pre2 (72x)