[WIP] Fix flame initialization

[bigfooted]

Proposed Changes

Give a brief overview of your contribution here in a few sentences.
Fix flame initialization and improve warning and error messages. The flame initialization also needs a regression test, with a physical check on temperature (ignition).

• 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.

[bigfooted]

Thanks, @EvertBunschoten
This PR just needs a regression test now, where the residual output includes some kind of flame temperature at the outlet, so we can check that ignition was successful.