Read extra lines from the given filename. Each line is defined in
        one line with four coordinates: x1 y1 x2 y2. (x1,y1) denotes the
        starting point, (x2, y2) the end point of the line.

def read_extra_lines(self, filename):
        """Read extra lines from the given filename. Each line is defined in
        one line with four coordinates: x1 y1 x2 y2. (x1,y1) denotes the
        starting point, (x2, y2) the end point of the line.
        """
        data = np.atleast_2d(np.loadtxt(filename))
        for nr, coords in enumerate(data):
            p1 = [coords[0], coords[1]]
            p2 = [coords[2], coords[3]]
            index = self.add_line(p1, p2, self.char_lengths['extra_line'])
            self.ExtraLineIndices.append(index)