Colab tutorials (#367)

* Colab Button & Run added

tutorials/tutorial3/tutorial.ipynb

@@ -7,6 +7,8 @@
    "source": [
     "# Tutorial: Two dimensional Wave problem with hard constraint\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/tutorial3/tutorial.ipynb)\n",
+    "\n",
     "In this tutorial we present how to solve the wave equation using hard constraint PINNs. For doing so we will build a costum `torch` model and pass it to the `PINN` solver.\n",
     "\n",
     "First of all, some useful imports."
@@ -19,6 +21,15 @@
    "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",
     "import torch\n",
     "\n",
     "from pina.problem import SpatialProblem, TimeDependentProblem\n",