Create an assy_context smart transform, encapsulating the transform to go from one part occurrence in an assembly tree to another. Either from_part_occ or to_part_occ can be NULL_TAG, indicating that the transform goes to or from the root part occ of the specified part. If they are both non-NULL, they must be in the same part. If from_part_occ and to_part_occ are the same, including if they are both NULL_TAG, then no assy_context_xform is created, and NULL_TAG is returned in the relevant argument. If to_part_occ is not NULL_TAG, its prototype part has to be the part that contains object_in_part.