Implement multicore running in CI

Changes introduced

For processes that needed to be run in multicore mode athena would fail in the CI. Here we use the ATHENA_PROC_NUMBER printout from log.generate to propagate that to the CI job and use 4 cores (fixed) if the job needs to run in multicore mode.

Tests

Seemed to be running fine in: https://gitlab.cern.ch/atlas-physics/pmg/mcjoboptions/-/jobs/13887928 I killed that so we can test in !1161 (merged)

Issues resolved

Closes #