Use common shape to resolve buffer access

Pystencils assume that all fields have the same spatial shape. Thus the field access should also be resolved by one common field shape. This was violated in the GPU kernel creation and should be fixed with this MR

pystencils/gpucuda/kernelcreation.py

@@ -91,10 +91,8 @@ def create_cuda_kernel(assignments: Union[AssignmentCollection, NodeCollection],
 
     coord_mapping = {f.name: cell_idx_symbols for f in all_fields}
 
-    loop_strides = list(fields_without_buffers)[0].shape
-
     if any(FieldType.is_buffer(f) for f in all_fields):
-        resolve_buffer_accesses(ast, get_base_buffer_index(ast, indexing.coordinates, loop_strides), read_only_fields)
+        resolve_buffer_accesses(ast, get_base_buffer_index(ast, indexing.coordinates, common_shape), read_only_fields)
 
     resolve_field_accesses(ast, read_only_fields, field_to_base_pointer_info=base_pointer_info,
                            field_to_fixed_coordinates=coord_mapping)