[WIP] Fix multigrid agglomeration

[bigfooted]

Proposed Changes

Give a brief overview of your contribution here in a few sentences.

Implement multigrid agglomeration according to Nishikawa and Diskin.

• euler walls are working correctly.

• mpi still needs to be checked.

• 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.

In general, to fix commented‑out code, either remove it entirely or reinstate it as active code, and rely on version control history if you ever need to recover old implementations. If the intent is to document behavior, rewrite it as an explanatory comment rather than a compilable snippet.

For this case, the best fix without changing existing functionality is to delete the commented‑out declaration of factor on line 649. The active implementation already sets factor via base_damp, so removing the old, commented line will not alter behavior. No new methods, imports, or definitions are required; only the single line within CMultiGridIntegration::SetProlongated_Correction in SU2_CFD/src/integration/CMultiGridIntegration.cpp needs to be removed.

[bigfooted]

When MG is finalized, these will be initial values and will be adaptively changed.

In general, to fix commented‑out code you either (1) delete it outright if it is no longer needed, or (2) convert it into a clearly non-code comment (e.g., “Alternative formula: ...”) so that it is obviously documentation, not dormant logic. You avoid re-enabling it unless you are sure that behavior is desired and still correct.

For this specific case, the best low‑risk fix without changing existing functionality is to keep Coeff equal to 1.1 as it is now, and rewrite the commented pow(...) expression into a descriptive comment. That removes the appearance of commented‑out code while retaining the information about the alternative formula. Concretely, in Common/src/geometry/CMultiGridGeometry.cpp around line 722, replace the current line with a normal comment that explains that 1.1 is a fixed approximation and that the commented expression is the original adaptive formula. No imports or additional definitions are required.