TruthSvc logic and debugging updates (ATLASSIM-6361)

Fix for two bugs in the TruthSvc:

1. Attempt to access the GenEventBarcodes attribute using the wrong key ("GenEventBarcodes" rather than "barcodes"). This fixes the crash reported in ATLASSIM-6361.
2. Fix logic for how to update the GenEvent in the case that a particle with a pre-defined decay (i.e. a quasi-stable particle) undergoes an additional non-destructive interaction prior to being decayed. (This was spotted while debugging issue (1).) Unit tests updated for change in call pattern.

These changes will alter the simulation output in principle, but only in the case that the event contains a particle with a pre-defined decay which undergoes an additional non-destructive interaction prior to being decayed. This is quite rare in most samples, so I would not be surprised if the CI tests see no change in output.
Compilation tested for HepMC2 and HepMC3 builds.
Tagging @averbyts, @martindl