Add unsteady cht adjoint testcase

[TobiKattmann]

Proposed Changes

Added files

• README which explains the files and why they are there
• cfg files which contain notes how to be changed for primal/findiff and serious adjoint run
• findiff.py FADO script for finite differences
• postprocess.py to compare the gradients of DA and FD (only screen output)
• gmsh .geo file with which the mesh was created

parallel_reg_AD will fail until the Testcases PR 63 is merged.

Related Work

Depends on #1260 and contains all of those changes (i.e. best reviewed after that is merged .. or just look at the last commit :) ). Meant to be merged shortly after but before #1284 to have an additional safeguarded case.

PR Checklist

• I am submitting my contribution to the develop branch.
• 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).
• 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.

[lgtm-com]

This pull request introduces 1 alert when merging 3548413 into 7b1ad11 - view on LGTM.com

new alerts:

• 1 for Unused import

[pcarruscag]

👍 LGTM

[TobiKattmann]

Code Factor complains about 2 things which are negligible imo. Good'ol complex code in CSolver which I didn't even touch and wildcard import of FADO things from FADO import * in my gradient_comparison script for FADO.