A few small fixes in example notebooks

examples/jupyter/mg-mode-part-i.ipynb

@@ -414,7 +414,7 @@
     }
    ],
    "source": [
-    "root_universe.plot(center=(0., 0., 0.), width=(3 * 21.42, 3 * 21.42), pixels=(500, 500),\n",
+    "root_universe.plot(origin=(0., 0., 0.), width=(3 * 21.42, 3 * 21.42), pixels=(500, 500),\n",
     "                   color_by='material')"
    ]
   },

examples/jupyter/mg-mode-part-ii.ipynb

@@ -306,7 +306,7 @@
     }
    ],
    "source": [
-    "root_universe.plot(center=(0., 0., 0.), width=(21.42, 21.42), pixels=(500, 500), color_by='material')"
+    "root_universe.plot(origin=(0., 0., 0.), width=(21.42, 21.42), pixels=(500, 500), color_by='material')"
    ]
   },
   {

examples/jupyter/mg-mode-part-iii.ipynb

@@ -412,7 +412,7 @@
     }
    ],
    "source": [
-    "universes['Root'].plot(center=(length / 2., length / 2., 0.),\n",
+    "universes['Root'].plot(origin=(length / 2., length / 2., 0.),\n",
     "                       pixels=(500, 500), width=(length, length),\n",
     "                       color_by='material',\n",
     "                       colors={materials['Fuel']: (1., 0., 0.),\n",
@@ -1053,7 +1053,7 @@
     }
    ],
    "source": [
-    "geometry_file.root_universe.plot(center=(length / 2., length / 2., 0.),\n",
+    "geometry_file.root_universe.plot(origin=(length / 2., length / 2., 0.),\n",
     "                       pixels=(300, 300), width=(length, length),\n",
     "                       color_by='material')"
    ]