Colab tutorials (#367)

* Colab Button & Run added

docs/source/_rst/tutorials/tutorial6/tutorial.rst

@@ -1,6 +1,11 @@
 Tutorial: Building custom geometries with PINA ``Location`` class
 =================================================================
 
+|Open In Colab|
+
+.. |Open In Colab| image:: https://colab.research.google.com/assets/colab-badge.svg
+   :target: https://colab.research.google.com/github/mathLab/PINA/blob/master/tutorials/tutorial6/tutorial.ipynb
+
 In this tutorial we will show how to use geometries in PINA.
 Specifically, the tutorial will include how to create geometries and how
 to visualize them. The topics covered are:
@@ -13,6 +18,15 @@ We import the relevant modules first.
 
 .. code:: ipython3
 
+    ## routine needed to run the notebook on Google Colab
+    try:
+      import google.colab
+      IN_COLAB = True
+    except:
+      IN_COLAB = False
+    if IN_COLAB:
+      !pip install "pina-mathlab"
+
     import matplotlib.pyplot as plt
     from pina.geometry import EllipsoidDomain, Difference, CartesianDomain, Union, SimplexDomain
     from pina.label_tensor import LabelTensor