Add V&V for the 2D Backward facing step

[SchitingLuo]

Proposed Changes

V&V Testcase for 2D BFS of SST.

Related Work

Validation Suite for SU2

PR Checklist

• [ X ] 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.

[bigfooted]

The V&V cases go here (meshes, scripts, everything to reproduce):
https://github.com/su2code/VandV
The description of the V&V (images and textpage) go on the website:
https://github.com/su2code/su2code.github.io
you can clone and run locally to test if it renders correctly (I use the command bundle exec jekyll serve to start the server locally)
The regression tests go here (meshes):
https://github.com/su2code/TestCases
and here (config and regression results):
https://github.com/su2code/SU2