Allow the resetting of the gen particle momentum in ISF only at start of event
This MR fixes a second bug found in the investigations of https://its.cern.ch/jira/browse/ATLPHYSVAL-630 that was exposed by !32419 (merged). While !32419 (merged) fixed the muon reco momentum, this MR will now fix the truth momentum.
Edited by Michael Duehrssen-Debling