Simplify OMP min/max reductions, cleanup raw indexing into CSysVector

[pcarruscag]

Proposed Changes

OpenMP 5.1 allows more efficient min/max than how we were doing them.
Also cleaned up CSysVector[i * n + j] to just CSysVector(i, j)

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.