add vertex by coordinate and uniq name
        x y z is coordinates of vertex
        name is uniq name to refer the vertex
        returns Vertex object whici is added.

def add_vertex(self, x, y, z, name):
        """add vertex by coordinate and uniq name
        x y z is coordinates of vertex
        name is uniq name to refer the vertex
        returns Vertex object whici is added.
        """
        self.vertices[name] = Vertex(x, y, z, name)
        return self.vertices[name]