Reduce the usage of barcodes in the TruthIsolations
Hi @pclark, this MR is, perhaps one of the few remaining examples where one can eliminate barcodes w/o many problems.
In the original code, the barcodes were used to enumerate particles. This means that no information on the particle history in simulation is used.
And barcode is just an id. To be compatible with the HepMC3 branch I use HepMC::barcode_or_id(), which is barcode() for HepMC2 branch, but id() for master/HepMC3.
Tag @jchapman