def initPos(self, startpos=0.0):
        """ initialize the elements position [m] in lattice, the starting
            point is 0 [m] for the first element by default.

            :param startpos: starting point, 0 [m] by default
        """
        spos = startpos
        for ele in self._lattice_eleobjlist:
            # print("{name:<10s}: {pos:<10.3f}".format(name=ele.name, pos=spos))
            ele.setPosition(spos)
            spos += ele.getLength()
initialize the elements position [m] in lattice, the starting
            point is 0 [m] for the first element by default.

            :param startpos: starting point, 0 [m] by default