Update CMake boost python logic and std::filesystem usage

See merge request walberla/walberla!319

The two-argument version was removed late in the standardization process

We need to have at least one other required library, otherwise FindBoost will consider itself successful even if it didn't find anything.
Also, add Python 3.8 support by using the Python version numbers as library version numbers.

Adapting flake8 formatig for python files

See merge request walberla/walberla!317

[BUGFIX] caching of buffers can lead to dead locks

See merge request walberla/walberla!318

If load balancing changes the neighborhood, caching of receive
buffers can lead to dead locks.

Unify python code style

See merge request walberla/walberla!306

fixed smaller formatting issues in mesa_pd

See merge request walberla/walberla!311

OpenMP support is now switchable within the generation script

See merge request walberla/walberla!313

[API] improved interface of merge sqlitedb script

See merge request walberla/walberla!314

Since MSVC only has beta support for OpenMP 2.0 firstprivate
cannot be used for these compilers. However, firstprivate
is necessary for a reasonably safe implementation of OpenMP
in MESA-PD. The solution is to move OpenMP support to the
generator and disable OpenMP for the checked in version.
The waLBerla OpenMP guards are removed as they are no longer
needed. It is now assumed that if MESA-PD is generated with
OpenMP support it is about to be used. If OpenMP is disabled
with CMake warnings will pop up about undefined pragmas.

Multiple field swaps test case

Closes #112

See merge request walberla/walberla!309

Support of convex meshes for MESA-PD

See merge request walberla/walberla!289

Phasefield allen cahn

See merge request walberla/walberla!302

fix coverage flag such that it can be used by apps

See merge request walberla/walberla!310

update external json lib

See merge request walberla/walberla!308

coverage: print summary and generate cobertura report

See merge request walberla/walberla!307

Enable CI on github

Closes #101

See merge request walberla/walberla!295

adjust coverage flags in CMakeLists.txt and fix coverage job in CI

See merge request walberla/walberla!305

pe sphere VTK output: add angular velocity

See merge request walberla/walberla!304

Fix python_coupling on recent macOS

See merge request walberla/walberla!303

Resolve "Bug in mesa-pd kernel generation"

Closes #122

See merge request walberla/walberla!298