Fix IDPVM truth pT cut
This MR increases the IDPVM truth particle pT cut from 400 to 500 MeV, to match the cut used in track reconstruction (c.f. https://twiki.cern.ch/twiki/bin/viewauth/AtlasProtected/TrackingCPRecsEarly2018). The mismatch had a noticeable effect on efficiencies for samples such as minbias, where the performance is dominated by the low pT regime. See attached plot: 
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