revive temperature-based ppm tracing

[zingale]

PR summary

PR motivation

PR checklist

• [ ] test suite needs to be run on this PR
• [ ] this PR will change answers in the test suite to more than roundoff level
• [ ] all newly-added functions have docstrings as per the coding conventions
• [ ] the CHANGES file has been updated, if appropriate
• [ ] if appropriate, this change is described in the docs

[zingale]

this seems to work, but I need to

1. figure out the T source term
2. figure out the T geometry term

[zingale]

this is part of #781