Proposed Changes

Give a brief overview of your contribution here in a few sentences.

Adapting and fixing mixing density model when species transport equations are being solved.
The species dependent properties will be added in 3 PRs:
1/3 : mixture density model (this PR)
2/3 : mixture viscosity + heat capacity
3/3 : mixture mass diffusivity + thermal conductivity

Related Work

Resolve any issues (bug fix or feature request), note any related PRs, or mention interactions with the work of others, if any.
Mark Heimgartner implemented the MIXTURE_FLUID model during his master thesis at Bosch. In this pull request, we aim to adapt his implementation to the latest version of develop branch, fixing some implementations that we think were causing numerical issues in his original implementation.

EDIT: Here, the mixture density will be implemented, based on the ideal gas law (computing mixture molecular weights).

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 the '-Wall -Wextra -Wno-unused-parameter -Wno-empty-body' compiler flags, or simply --warnlevel=2 when using meson).
• [ X] My contribution is commented and consistent with SU2 style.
• 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.