OpenMPI 3.0 by default does not allow running with more processes than CPUs

Also added wrapper for Metis & ParMetis to prevent it from polluting the global name space

- normal CMake Python search can be used if the python interpreter is
  searched before the searching the libraries

- nvcc also printed warnings in system libs like boost

FindCUDA apparently can't handle CMake "generator expressions"
these are removed before calling FindCUDA and restored afterwards

- default python is the one reported by python3-config

two versions available now via CMake switch:
   - by default the Python module is built without LBM module, since
     lbmpy can be used for this. Reduces compile time substantially.
   - if the LBM is needed in python, for example for the lbmpy testsuite
     it can be included by setting the CMake switch
     WALBERLA_BUILD_WITH_PYTHON_LBM

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/*'