Reactivate multi-threading on MacOS wheels and update code to C++20 standards

[fteicht]

This PR:

• Reactivate multi-threading when building MacOS wheels, which was deactivated because of cmake not able anymore to find OpenMP on MacOs without hints on where the libomp library is ;
• Re-separate pytests in separate processes, one per pytest folder, in order to avoid launching all the pytests in a same process and having OpenMP library version conflicts with some distributions ;
• Move the C++ codebase to c++20 standards which most dependent packages under the cpp/sdk folder are following.

[nhuet]

@fteicht Can you develop why you need to split again the tests ? (by the way you forgot the flight_planning and domains tests: that is the risk, when specifying manually the subdirectories)

[fteicht]

@fteicht Can you develop why you need to split again the tests ? (by the way you forgot the flight_planning and domains tests: that is the risk, when specifying manually the subdirectories)

@nhuet If you run pytest by calling it on the parent folder of all the tests, then it runs all the tests in a same process. Unfortunately, with some distributions and especially MacOS, different versions of OpenMP are linked against the C++ hub library provided by scikit-decide, and against deep learning libraries like tensorflow provided in the dependent pip packages. It results in a crash of the whole pytest process. It does not happen when we separate the calls to pytest among the different subfolders, because then the pytests run in different processes, each process linking against a single (different) OpenMP library.

One solution could be to write a bash script that calls the pytests on the different separate test sub-folders. Then we'd call this script in the build and release scripts. It's not as robust as a single call to pytest on the parent test folder, but it's still more robust than calling the series of pytest calls multiple times in the build and release scripts.

[nhuet]

If I understand well, what is important is to call the cpp/ subfolders in an other process than the other directories (python/) ? Is it correct (and sufficient) ?

[fteicht]

Yes you got it right @nhuet

[g-poveda]

lgtm !