code: def _create(self, monomer, mon_vector):
          
          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)
docs: 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