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

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