Fix import of modelchain_example - previous import fails when notebook is run using pytest_notebook

Author: birgits

example/turbine_cluster_modelchain_example.ipynb

@@ -39,7 +39,7 @@
    "source": [
     "import pandas as pd\n",
     "\n",
-    "import modelchain_example as mc_e\n",
+    "from example import modelchain_example as mc_e\n",
     "from windpowerlib import TurbineClusterModelChain, WindTurbineCluster, WindFarm\n",
     "\n",
     "import logging\n",