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/boundaryhandling.py:102: TypeError

None is not a type! ParallelDataHandling is None if waLBerla is not installed

pystencils/boundaries/boundaryhandling.py

@@ -99,7 +99,7 @@ class BoundaryHandling:
             gpu_version = gpu_version.boundary_object_to_index_list
             cpu_version = cpu_version.boundary_object_to_index_list
 
-            if isinstance(self.data_handling, ParallelDataHandling):
+            if ParallelDataHandling and isinstance(self.data_handling, ParallelDataHandling):
                 array_handler = PyCudaArrayHandler()
             else:
                 array_handler = self.data_handling.array_handler