CountHepMC: adjust the value of m_nCount in the MP mode after forking

Vakhtang Tsulaiarequested to merge
tsulaia/athena:main-counthepmc into main

This fix ensures that the entire MP job produced the number of events consistent with the value of maxEvents, while evenly distributing the workload between the worker processes

Related to AGENE-2322

Edited by Vakhtang Tsulaia

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