Updates to Jacobian computations

[VictoriaGuenter]

This is a small Jacobian computation update to make things work in parallel (for Summa Actors)

Make sure all the relevant boxes are checked (and only check the box if you actually completed the step):

• Closes #xxx (identify the issue associated with this PR)
• Code passes standard test cases (results are either bit-for-bit identical, or differences are explained in the PR comment)
• New tests added (describe which tests were performed to test the changes)
• Science test figures (add figures to PR comment and describe the tests)
• Checked that the new code conforms to the SUMMA coding conventions
• Describe the change in the release notes (use ./summa/docs/whats-new.md)