Add dummy HITS merge step after serial simulation when comparing to AthenaMP output

AthenaMP jobs produce N separate HITS files, which are then merged into a single output. This can cause rounding differences in SiHitCollections and TRT_HitCollections. In order to restore agreement with the output of serial Athena jobs, we add a dummmy HITS merge step after serial simulation job. This will make the rounding match. Should fix ATLASSIM-6038