missing cuda tag now implies WALBERLA_BUILD_WITH_CUDA=OFF

- fix find_package(MPI) when WARNING_ERROR is defined and recent version of CMake is used
- fix python_coupling Manager::addPath

Updated conda packages to cond build > 3.1.4 using host section
See https://conda.io/docs/user-guide/tasks/build-packages/define-metadata.html#host

- fixed problem with new conda by using new compiler mechanism
- added test file that is checked before upload
- open mesh recipe fixed
- force downgrade for conda-build, new tested version(3.2.1) of conda-build leads to an empty
  package and then failing test.. perhaps it is fixed with newer conda-build versions?

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/*'