docs: def get_sets_index(self):
          """
          Returns a one dimension array with the Place where the points 
          belong.
          
          :returns: The ``pyny.Place`` where the points belong.
          :rtype: list of int
          """
          index = []
          for i, place in enumerate(self):
              index.append(np.ones(place.set_of_points.shape[0])*i)
          return np.concatenate(index).astype(int)