Mtot as primary, not Msun

[awilson110]

Fixes #447 .

Replaced the primary to be Mtot instead of Msun

Review Checklist for Source Code Changes

• Does pip install still work?
• Have you written a unit test for any new functions?
• Do all the units tests run successfully?
• Does Layup run successfully on a test set of input files/databases?
• Have you used black on the files you have updated to confirm python programming style guide enforcement?

[hannorein]

Hi @awilson110, this looks good. I think this should work.

[matthewholman]

Correct fix for #447 — rebound.Particle(m=Mtot) is the barycenter at the origin, so the reported elements are now genuinely barycentric (μ = GM_total, position relative to the SSB), and the demo_comet_expected.csv fixtures are regenerated to match.

One thing worth confirming: the pure-REBOUND fallback (include_assist=False, previously sim.particles[0]) now also uses the origin+Mtot primary. That's right as long as simulation_convert_to_rebound leaves the sim in the SSB frame — but that branch only fires for comets past the ASSIST timeframe, so the fixtures probably don't exercise it. A quick check would close it.

Minor: tests/layup/test_layup_configs.py drops one line near the top — intentional, or a stray edit?

[awilson110]

Correct fix for #447 — rebound.Particle(m=Mtot) is the barycenter at the origin, so the reported elements are now genuinely barycentric (μ = GM_total, position relative to the SSB), and the demo_comet_expected.csv fixtures are regenerated to match.

One thing worth confirming: the pure-REBOUND fallback (include_assist=False, previously sim.particles[0]) now also uses the origin+Mtot primary. That's right as long as simulation_convert_to_rebound leaves the sim in the SSB frame — but that branch only fires for comets past the ASSIST timeframe, so the fixtures probably don't exercise it. A quick check would close it.

@hannorein would you be able to confirm that the function simulation_convert_to_rebound keeps the simulation in the barycentric frame? I'd imagine it does, but good to check

Minor: tests/layup/test_layup_configs.py drops one line near the top — intentional, or a stray edit?

That was a mistake, I can remove it if needed

[hannorein]

Yes, simulation_convert_to_rebound does not change the frame.

[matthewholman]

Looks good