Add wrappers for CTU advance source terms

[maxpkatz]

PR summary

For CTU we can hardcode which MultiFabs will be used in the old and new source term calls, so we add wrappers to simplify the logic in the main driver routine.

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]

quick tests pass: http://groot.astro.sunysb.edu/Castro/test-suite/gfortran/2023-06-17-002/index.html