Show error message when FLUID_MIXTURE is used with KIND_SCALAR_MODEL = NONE

[Cristopher-Morales]

Proposed Changes

This pull request restricts the use of the FLUID_MIXTURE model only when the KIND_SCALAR_MODEL is SPECIES_TRANSPORT.

Related Work

Reported in CDF-Online https://www.cfd-online.com/Forums/su2/257519-kind_scalar_model-mixture.html

PR Checklist

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• [ x] I am submitting my contribution to the develop branch.
• [ x] My contribution generates no new compiler warnings (try with --warnlevel=3 when using meson).
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• [ ] 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.