Fix bug related to ParallelDataHandling

    def to_gpu(gpu_version, cpu_version):
        gpu_version = gpu_version.boundary_object_to_index_list
        cpu_version = cpu_version.boundary_object_to_index_list

>       if isinstance(self.data_handling, ParallelDataHandling):
E       TypeError: isinstance() arg 2 must be a type or tuple of types

../pystencils/pystencils/boundaries/ TypeError

None is not a type! ParallelDataHandling is None if `waLBerla` is not
9 jobs for fix-ParallelDataHandling-bug! in 6 minutes and 3 seconds (queued for 1 second)