External apllications which did not set the C++ standard to 14 in their CMakeLists.txt failed to build. This change allows them to build without further user actions.

- one python file can now generate multiple source files

- Add generator files to targets to show them in VS solutions
- mark generated files as such in CMake

If you have access to the old closed source waLBerla repository, you may restore the complete history with the following command
git fetch git@i10git.cs.fau.de:software/walberla.git 'refs/replace/*:refs/replace/*'