Migrate current callers to level-indexed refinement API

tests/validation/multigrid/test_fine_surface_gpu.py

@@ -81,7 +81,7 @@ def run_blocked_patch(speed):
                                               level_objects=[(), (source,)],
                                               observe_raw_exchange=True) as backend:
             c, f = backend._levels
-            flags = backend.fine_boundary.flags.reshape(40, 40)
+            flags = backend.boundaries[1].flags.reshape(40, 40)
             active = np.zeros((40, 40), bool)
             active[4:-4, 4:-4] = True
             active &= ((flags & FLUID) != 0) & ((flags & SOLID) == 0)
@@ -101,7 +101,7 @@ def run_blocked_patch(speed):
             with patch.object(f, "kernel", fine):
                 backend.step(2)
             assert len(traces) == 4 and backend.step_count == 2
-            mass = backend.read_raw_exchange(0)["total"]["mass_into_fluid"]
+            mass = backend.read_raw_exchange(0, level=1)["total"]["mass_into_fluid"]
             return traces, roi, backend.download(), float(mass)