Colab tutorials (#367)

* Colab Button & Run added

tutorials/tutorial12/tutorial.ipynb

@@ -4,7 +4,9 @@
    "cell_type": "markdown",
    "metadata": {},
    "source": [
-    "# Tutorial: The `Equation` Class"
+    "# Tutorial: The `Equation` Class\n",
+    "\n",
+    "[![Open In Colab](https://colab.research.google.com/assets/colab-badge.svg)](https://colab.research.google.com/github/mathLab/PINA/blob/master/tutorials/tutorial12/tutorial.ipynb)"
    ]
   },
   {
@@ -49,6 +51,15 @@
    "metadata": {},
    "outputs": [],
    "source": [
+    "## routine needed to run the notebook on Google Colab\n",
+    "try:\n",
+    "  import google.colab\n",
+    "  IN_COLAB = True\n",
+    "except:\n",
+    "  IN_COLAB = False\n",
+    "if IN_COLAB:\n",
+    "  !pip install \"pina-mathlab\"\n",
+    "\n",
     "#useful imports\n",
     "from pina.problem import SpatialProblem, TimeDependentProblem\n",
     "from pina.equation import Equation, FixedValue, FixedGradient, FixedFlux\n",