- recent GCC version produce error when "-isystem /usr/include" is added
  to command line. Error is: '#include_next <math.h>' not found
- the removed "hack" added the cuda include dir with "-isystem" to
  the CUDA compiler arguments to prevent warnings from boost header
  -> currently there seem to be no warnings generated from boost headers
     so "hack" is removed

added filter to exclude external libraries

- NearestNeighborFieldInterpolator::get needs to break out of all loops, just not the current one. While the test seemed to succeed anyway, it crashed with corrupted memory during exit when compiled with Intel 17 in Release mode.
- FftTest should not use thread-unsafe RNG in parallel section. Otherwise the Intel 16 compiler crashes.
- Revert to -openmp on Intel < 16.0.3, which crashes while processing some of our OpenMP loops (e.g. BoundaryHandling::treatDirection).

Also adds OpenMP support for macOS (requires Xcode 7.0, CMake 3.12 and separate libomp from MacPorts/Homebrew)

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

- Need to find Boost library path before we search for the Python libraries, even if we don't need any other libraries
- std::experimental::filesystem::current_path is broken in libstdc++ 5.4 and lower

API changes in blockforest::PhantomBlockForest, blockforest::loadbalancing, pe::amr::weight_assignment
mpi can now only send/receive walberla::optional instead of boost::optional

API change for vtk::VTKOutput::getFilenames

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