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 seems to be working (can be improved)

• OMP seems to be working (can be improved)

• multigrid does not crash anymore

• 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“commented-out code” issues you either (a) re-enable the code if it is still needed, or (b) delete it entirely if it is obsolete or merely serving as a reminder. If a conceptual description is useful for future readers, that should be expressed as a natural-language comment rather than as disabled code.

For this specific case in SU2_CFD/src/integration/CMultiGridIntegration.cpp, the two lines:

//solver_fine->InitiateComms(geometry_fine, config, MPI_QUANTITIES::SOLUTION);
//solver_fine->CompleteComms(geometry_fine, config, MPI_QUANTITIES::SOLUTION);

are clearly commented-out function calls. The best fix that does not change existing functionality is to remove these two lines entirely while leaving the surrounding smoothing logic unchanged. The preceding comment “/--- MPI sync after RK stage to ensure halos have updated solution for next smoothing iteration ---/” already explains the intended behavior; the extra line // nijso: check if this can be removed now. is more of a personal note. We can keep or slightly rephrase it as a neutral TODO-style comment without any code; to minimize behavioral changes, we will simply keep it. No new methods, imports, or definitions are required.

Concretely:

• Edit SU2_CFD/src/integration/CMultiGridIntegration.cpp.
• In the loop where post-smoothing is performed (around line 562), delete the two commented-out lines containing solver_fine->InitiateComms and solver_fine->CompleteComms.
• Leave all other code and comments intact.