set number of Pythia8 events to 1 (both as Pythia8 param and config param of the transform)

Ewelina Maria Lobodzinskarequested to merge
ewelina/athena:21.6-mp into 21.6

https://its.cern.ch/jira/browse/AGENE-1794 set number of Pythia8 events to 1 (both as Pythia8 param and config param of the transform)

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