Refactor of the NEMO solvers: shared helpers and removal of unreachable code

[rois1995]

Proposed Changes

A clean-up of the two NEMO solver classes (CNEMOEulerSolver, CNEMONSSolver). It changes no result and adds no feature.

• Removes code that could never run. BC_Inlet and BC_HeatFluxCatalytic_Wall start with a "not operational" error, so the rest of their bodies was dead. Commented-out blocks are removed too.
• Replaces copied blocks with six small helpers, for the data passed to the numerics, the viscous flux at a boundary, and the velocity and energy treatment of the walls.
• Uses fixed-size local arrays where boundary conditions allocated memory at every call.
• Shares the reference values with CEulerSolver through one function in CFVMFlowSolverBase. This is the only change outside NEMO; it is one commit (222b7b64d4) and can be split off if you prefer.

The two NEMO solver files go from 3,404 to 2,728 lines.

Why it helps. The copies had drifted apart over time, and a correction made in one was missing in the others. With one helper per task, a fix is made once.

Why not build NEMO on CScalarSolver. I looked at this first. The scalar solver runs after the flow solver, as a separate system. In NEMO the pressure depends on the composition and the chemistry is stiff, so species, energy and flow are solved together on purpose. Splitting them would be a rewrite that changes every NEMO result, so I kept the coupled solver.

Checks, against develop built on the same machine: identical values for the 14 NEMO regression cases, for three extra NEMO cases (heat-flux wall, slip wall, viscous supersonic inlet) and for six standard compressible cases covering all REF_DIMENSIONALIZATION options. The NEMO unit tests pass. No warnings with --warnlevel=3 in the changed files, also in the AD configuration. Not checked: runs with Mutation++, OpenMP or AD.

Related Work

This is only the refactoring. While reading the code I collected a list of bugs and possible improvements in NEMO. None is addressed here on purpose: they will come in separate pull requests, grouped by area of the code, so that a change of results is never mixed with a change of structure. Where the old code had a defect, this pull request keeps the old behavior.

PR Checklist

• 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.

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https://claude.ai/code/session_01L7oxzVir4zENrpy3K3W6Fm