Add new example for adjoint fluid flow with custom mesh deformation, fix CGNS issue with compiler flags

[pcarruscag]

Proposed Changes

I'm also working on an unsteady version, but there is something wrong with the unsteady adjoint.

Related Work

Fixes #2058

PR Checklist

• 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.

[pcarruscag]

For completeness, unsteady adjoints do not work for this type of case due to a dependence of the solution on old coordinates (to compute grid velocities) and old volumes (for the dual time derivative) which we do not handle correctly.