fixed a bug where 1e5 used instead of 1e6 and modified to use MagicNumber constant instead

There was a hardcoded number 100000 instead of 1000000, meant suppress all regenerated Geant4 particles, i.e. >1e6 barcodes. This also removes pileup particles (normally suppressed/squashed to barcode = std::numeric_limits<int32_t>::max()).

Since we are in the process of trying to separate id and origin barcode usage, we've put a MagicNumber.h constant for now. Likely have to come back to this to use a function later.

@averbyts @lmijovic @asalzbur

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