Fix high multiplicity events production in EPOS
Small fix the memory crash in high multiplicity embedded events produced with EPOS. The bug is due to an event stacking procedure in Special.cpp which piles up 30 events by default. The fix gives access to the constant which set the number of piled-up events and set this value to 1 in EPOS.py
For more details, see LHCBGAUSS-1615.