colab: make setup cell more robust

Author: Masood-Akram

Python/01_Image_Basics.ipynb

@@ -23,12 +23,21 @@
    "outputs": [],
    "source": [
     "# Setup to enable running the notebook in the Google Colab environment\n",
-    "import os\n",
+    "import pathlib\n",
     "\n",
-    "if \"COLAB_NOTEBOOK_ID\" in os.environ:\n",
-    "    !git clone https://github.com/InsightSoftwareConsortium/SimpleITK-Notebooks\n",
+    "def _in_colab() -> bool:\n",
+    "    try:\n",
+    "        import google.colab  # noqa: F401\n",
+    "        return True\n",
+    "    except Exception:\n",
+    "        return False\n",
+    "\n",
+    "if _in_colab():\n",
+    "    repo_dir = pathlib.Path(\"SimpleITK-Notebooks\")\n",
+    "    if not repo_dir.exists():\n",
+    "        !git clone --depth 1 https://github.com/InsightSoftwareConsortium/SimpleITK-Notebooks\n",
     "    %cd SimpleITK-Notebooks/Python\n",
-    "    !pip install -q -r requirements.txt"
+    "    !pip install -q -r requirements.txt\n"
    ]
   },
   {