calculating Mass accretion rate for SN post-processing

[mari2895]

PR Summary

PR Checklist

• [ ] Adds a test for any bugs fixed. Adds tests for new features.
• [x] Format your changes by calling scripts/bash/format.sh.
• [x] Explain what you did.

[mari2895]

As soon as tests pass here (and you confirm it works for you locally) I will merge.

All tests have passed, compiles, runs, produces reasonable output.