fix missing option

Author: Huangzizhou

test/test_basic.ipynb

@@ -21,6 +21,7 @@
     "with open(root + \"/initial-contact.json\", \"r\") as f:\n",
     "    config = json.load(f)\n",
     "\n",
+    "config[\"contact\"][\"use_convergent_formulation\"] = True\n",
     "config[\"root_path\"] = root + \"/initial-contact.json\"\n",
     "\n",
     "solver = pf.Solver()\n",
@@ -69,7 +70,7 @@
     "        sol = solver.step_in_time(sol, t0, dt, t+1)\n",
     "\n",
     "    t0 += dt\n",
-    "    solver.export_vtu(sol, np.zeros((0, 0)), t0, dt, \"step_\" + str(i) + \".vtu\")\n"
+    "    solver.export_vtu(\"step_\" + str(i) + \".vtu\", sol, np.zeros((0, 0)), t0, dt)\n"
    ]
   },
   {