Read extra nodes in. Format: x y

        What happens if we add nodes on the boundaries, which are not included
        in the boundaries?

def read_extra_nodes(self, filename):
        """Read extra nodes in. Format: x y

        What happens if we add nodes on the boundaries, which are not included
        in the boundaries?
        """
        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)