Compare NEMO freestream mass fractions against a roundoff tolerance

[bellonarts]

Proposed Changes

CSU2TCLib validates the user-supplied freestream composition with if (mf != 1.0), an exact floating-point equality on an accumulated sum, at four gas-model sites (ARGON, N2, AIR-5, AIR-7). A physically exact composition need not sum to bit-exact unity in double precision. For example

GAS_COMPOSITION= ( 0.999, 0.00025, 0.00025, 0.00025, 0.00025 )

sums to 1 - 1.1e-16 and is rejected with "Intial gas mass fractions do not sum to 1!", which reads to the user as a typo in a composition that visibly sums to one. Any case seeding trace species can hit this.

This PR compares against a 1e-10 roundoff tolerance and reports the offending sum in the message. The tolerance sits about six orders above the roundoff in the example (1.1e-16) and four orders below the 1.e-6 already used for the same "is this roundoff?" question in Common/src/fem/fem_geometry_structure.cpp, so genuine input errors are still rejected. The constant is constexpr passivedouble per the convention in option_structure.hpp, and the message uses SU2_TYPE::GetValue so the change is safe under AD builds. The sum is printed with 16 significant digits through a small static helper shared by the four sites, because to_string would print 1.000000 for any sum that fails the tolerance by less than 5e-7; a composition off by 3e-7 now reports sum = 0.9999997 and a real typo reports sum = 0.99. The pre-existing "Intial" typo is corrected in the same lines.

Verification with actual solver launches on the public viscous-cone mesh (ITER= 0, exit code as the verdict): the roundoff-unity AIR-5 and AIR-7 compositions are now accepted; exact compositions for ARGON, N2, AIR-5 and AIR-7 remain accepted; real typos such as ( 0.75, 0.24, 0, 0, 0 ) remain rejected for every gas model. The regression baselines are unchanged, serial and MPI, so no regression vectors are modified.

The contributor entry is added to AUTHORS.md. A dedicated constructor unit test was removed in response to maintainer review; the acceptance/rejection matrix above exercises the behavior through actual solver launches.

Related Work

The confusing failure mode may account for some reports of NEMO setup difficulty; it surfaces whenever trace species are seeded rather than left at exact zero. No open issue or PR is known to cover it.

PR Checklist

Put an X by all that apply. You can fill this out after submitting the PR. If you have any questions, don't hesitate to ask! We want to help. These a guide for you to know what the reviewers will be looking for in your contribution.

• 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.
• A dedicated unit test is not included at maintainer request; actual solver launches cover accepted roundoff and rejected invalid compositions.
• Documentation is not applicable; this corrects input validation and its diagnostic without changing a configuration keyword or public API.