Fix create_cuda_kernel to allow CUSTOM_FIELDS to have a different size

The CPU backend allows Fields with FieldType.CUSTOM to have a differnt spatial_shape. The GPU backend should also.

pystencils/gpucuda/kernelcreation.py

@@ -20,7 +20,7 @@ def create_cuda_kernel(assignments,
     all_fields = fields_read.union(fields_written)
     read_only_fields = set([f.name for f in fields_read - fields_written])
 
-    buffers = set([f for f in all_fields if FieldType.is_buffer(f)])
+    buffers = set([f for f in all_fields if FieldType.is_buffer(f) or FieldType.is_custom(f)])
     fields_without_buffers = all_fields - buffers
 
     field_accesses = set()