Add a boundary line

def add_boundary(self, p1, p2, btype):
        """
        Add a boundary line
        """
        index = self.add_line(p1, p2, self.char_lengths['boundary'])
        # self.Boundaries.append((p1_id,p2_id,btype))
        self.BoundaryIndices.append(index)
        self.Boundaries.append((p1, p2, btype))