create the polymer from the monomer

        Args:
            monomer (Molecule)
            mon_vector (numpy.array): molecule vector that starts from the
                start atom index to the end atom index

def _create(self, monomer, mon_vector):
        """
        create the polymer from the monomer

        Args:
            monomer (Molecule)
            mon_vector (numpy.array): molecule vector that starts from the
                start atom index to the end atom index
        """
        while self.length != (self.n_units-1):
            if self.linear_chain:
                move_direction = np.array(mon_vector) / np.linalg.norm(mon_vector)
            else:
                move_direction = self._next_move_direction()
            self._add_monomer(monomer.copy(), mon_vector, move_direction)