Generalize stream only kernel

There exists a create_stream_pull_only_kernel. This could be generalized for all Access patterns

lbmpy/updatekernels.py

@@ -56,7 +56,8 @@ def create_lbm_kernel(collision_rule, input_field, output_field, accessor):
 
 
 def create_stream_only_kernel(stencil, numpy_arr=None, src_field_name="src", dst_field_name="dst",
-                              generic_layout='numpy', generic_field_type=np.float64, accessor=None):
+                              generic_layout='numpy', generic_field_type=np.float64,
+                              accessor=StreamPullTwoFieldsAccessor()):
     """Creates a stream kernel, without collision.
 
     Args:
@@ -72,10 +73,6 @@ def create_stream_only_kernel(stencil, numpy_arr=None, src_field_name="src", dst
     Returns:
         AssignmentCollection of the stream only update rule
     """
-
-    if accessor is None:
-        accessor = StreamPullTwoFieldsAccessor()
-
     dim = len(stencil[0])
     if numpy_arr is None:
         src = Field.create_generic(src_field_name, dim, index_shape=(len(stencil),),