pywrapper - custom source terms for all solvers

[bigfooted]

Proposed Changes

Introduce custom source terms for all solvers through the python wrapper.

• reproduces the laminar buoyancy driven cavity testcase
• can simulate a turbulent premixed flame using turbulent flamespeed closure (TFC)

For this PR, we introduce only the custom source terms, other enhancements that were mentioned will be in separate PR's.

PR Checklist

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• I am submitting my contribution to the develop branch.
• My contribution generates no new compiler warnings (try with --warnlevel=3 when using meson).
• My contribution is commented and consistent with SU2 style (https://su2code.github.io/docs_v7/Style-Guide/).
• I used the pre-commit hook to prevent dirty commits and used pre-commit run --all to format old commits.
• I have added a test case that demonstrates my contribution, if necessary.
• I have updated appropriate documentation (Tutorials, Docs Page, config_template.cpp), if necessary.