Add sum_of_Vh function

This function computes the sum of the components of a DiscreteFunctionP0Vector into a DiscreteFunctionP0

doc/userdoc.org

@@ -3406,6 +3406,19 @@ and the return value is an $\mathbb{R}^{3\times3}$ matrix.
   return value is $$\sum_{j\in\mathcal{J}} A_j.$$
 
 
+***** Utilities
+
+****** ~sum_to_Vh: Vh -> Vh~
+
+This function it computes the sum of all the components of
+$\vec{\mathbb{P}}_0(\mathbb{R})$ function and stores the result into a
+$\mathbb{P}_0(\mathbb{R})$.
+
+****** ~vectorize: (Vh) -> Vh~
+
+This function creates a $\vec{\mathbb{P}}_0(\mathbb{R})$ function using
+a tuple of $\mathbb{P}_0(\mathbb{R})$.
+
 ***** Interpolation and integration functions
 
 These functions are ways to define discrete functions from analytic