Migrate current callers to level-indexed refinement API

tests/validation/multigrid/test_refinement_followup.py

@@ -304,7 +304,7 @@ class NormalBuildGPU(unittest.TestCase):
                             if not (x0 <= sx < x1 and y0 <= sy < y1):
                                 corners.add(target)
                 self.assertEqual(len(corners), 8)
-                self.assertEqual(set(backend.patch.coalesce_coarse.tolist()), targets)
+                self.assertEqual(set(backend.hierarchy.pairs[0].coalesce_coarse.tolist()), targets)
                 backend.initialize(*initial)
                 before = discrete_mass(initial)
                 for step in range(1, 9):
@@ -383,8 +383,8 @@ class NormalBuildGPU(unittest.TestCase):
         expected = scalar_interval(*[a.astype(np.float64) for a in initial], bounds)
         with CudaContext(0) as context, FixedPatchStepper(cfg, context=context, refinements=[bounds],
                                                              level_blocks=[(13, 17), (14, 16)]) as backend:
-            self.assertEqual(backend.patch.fine_shape, (14, 16))
-            p = backend.patch
+            self.assertEqual(backend.hierarchy.pairs[0].fine_shape, (14, 16))
+            p = backend.hierarchy.pairs[0]
             allocated = 2 * 4 * sum(int(np.prod(level.layout.blocked_shape)) for level in backend._levels) \
                 + sum(a.nbytes for a in (p.ghost_fine, p.ghost_coarse, p.coalesce_coarse, p.coalesce_fine))
             self.assertLess(allocated, 128*1024)