Fix hepmc3/macro initialization in celer-g4

[sethrj]

@wdconinc encountered a few issues recently when trying to run celer-g4 from scratch.

• Mark .mac file setup as deprecated since we're trying to transition celer-g4 to JSON-only input
• Default secondary stack factor to 2; we're not enabling atomic relaxation by default we only need room for annihilation
• Wrap HepMC3 construction in exception conversion to catch errors while loading
• No longer set num_events since we're using the single-event model

[amandalund]

Thanks for the updates!

[github-actions]

Test summary

 3 882 files   5 991 suites   4m 14s ⏱️
 1 609 tests  1 580 ✅ 29 💤 0 ❌
19 907 runs  19 832 ✅ 75 💤 0 ❌

Results for commit aa0a6f7.