xAODtoHepMCTool: fixed a bug where 1e5 used instead of 1e6

Merged Philip Clark requested to merge pclark/athena:23.0 into 23.0

There was a hardcoded number 100000 instead of 1000000, we think this was to suppress all regenerated Geant4, i.e. >1e6 barcodes, and pileup particles which are currently normally suppresssed (and have barcode = std::numeric_limits<int32_t>::max()).

This should now output only particles with barcodes <1e6. We've also used HepMC::SIM_REGENERATION_INCREMENT to help avoid these bugs and help when we phase out MagicNumber usage in barcodes to use origin instead.

@ewelina @jchapman @averbyts

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