code: def get_element_centroids(self):
          
          centroids = np.vstack((
              np.mean(self.grid['x'], axis=1), np.mean(self.grid['z'], axis=1)
          )).T
  
          return centroids
docs: return the central points of all elements
  
          Returns
          -------
          Nx2 array
              x/z coordinates for all (N) elements