Adding thermodynamics pressure as a user value for FLUID_MIXTURE model

[Cristopher-Morales]

Proposed Changes

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

The thermodynamic pressure influences the density at the inlet boundaries, especially when the mixture consists of more than 2 gases. So, with this pull request, we aim to obtain the pressure_thermodynamic as a user value or a default value, as 101325 [pa]. and based on this value compute the density. This is consistent with the low-mach approximation used in combustion.
I can add a test case if it is needed.

Related Work

Resolve any issues (bug fix or feature request), note any related PRs, or mention interactions with the work of others, if any.
This is pull request is related to #1826.

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 are 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 have added a test case that demonstrates my contribution, if necessary.
• I have updated appropriate documentation (Tutorials, Docs Page, config_template.cpp), if necessary.