Colab tutorials (#367)

* Colab Button & Run added

docs/source/_rst/tutorials/tutorial12/tutorial.rst

@@ -1,6 +1,11 @@
 Tutorial: The ``Equation`` 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/tutorial12/tutorial.ipynb
+
 In this tutorial, we will show how to use the ``Equation`` Class in
 PINA. Specifically, we will see how use the Class and its inherited
 classes to enforce residuals minimization in PINNs.
@@ -30,6 +35,15 @@ class.
 
 .. 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"
+    
     #useful imports
     from pina.problem import SpatialProblem, TimeDependentProblem
     from pina.equation import Equation, FixedValue, FixedGradient, FixedFlux