Migrate current callers to level-indexed refinement API

tests/unit/test_union_refinement.py

@@ -40,7 +40,7 @@ class Equivalence(unittest.TestCase):
                          table_sets(FixedPatchSet2D.from_config(CFG, [(8, 8, 20, 22)])))
 
     def test_separated_rectangles_reproduce_current_tables(self):
-        rects = [(4, 4, 10, 10), (16, 16, 26, 26)]  # gap 6 >= MIN_PATCH_GAP
+        rects = [(4, 4, 10, 10), (16, 16, 26, 26)]  # separated rectangles; union dependencies determine legality
         self.assertEqual(table_sets(union(*rects)),
                          table_sets(FixedPatchSet2D.from_config(CFG, rects)))