code: def read_extra_nodes(self, filename):
          
          data = np.atleast_2d(np.loadtxt(filename))
          for nr, pair in enumerate(data):
              index = self.get_point_id(pair, self.char_lengths['extra_node'])
              self.ExtraNodes.append(index)
docs: Read extra nodes in. Format: x y
  
          What happens if we add nodes on the boundaries, which are not included
          in the boundaries?