examples: Show strided sub-block gather in MPI data notebook

Now that data_gather handles a strided slice whose start falls inside a
subdomain, demonstrate it in the reading example
(f.data_gather(start=2, stop=8, step=2)).

Author: mloubout

examples/mpi/data_initialization.ipynb

@@ -310,12 +310,12 @@
     "myrank = grid.distributor.myrank\n",
     "\n",
     "# `data_gather` collects (a slice of) the global array onto one rank.\n",
-    "whole = f.data_gather(rank=0)                     # full global array\n",
-    "window = f.data_gather(start=2, stop=8, rank=0)   # contiguous sub-block\n",
-    "reverse = f.data_gather(step=-1, rank=0)          # reversed on every axis\n",
+    "whole = f.data_gather(rank=0)                            # full global array\n",
+    "window = f.data_gather(start=2, stop=8, step=2, rank=0)  # strided sub-block\n",
+    "reverse = f.data_gather(step=-1, rank=0)                 # reversed on every axis\n",
     "if myrank == 0:\n",
     "    assert np.all(whole == glb_a)\n",
-    "    assert np.all(window == glb_a[2:8, 2:8])\n",
+    "    assert np.all(window == glb_a[2:8:2, 2:8:2])\n",
     "    assert np.all(reverse == glb_a[::-1, ::-1])"
    ]
   },