extend the polymer molecule by adding a monomer along mon_vector direction

        Args:
            monomer (Molecule): monomer molecule
            mon_vector (numpy.array): monomer vector that points from head to tail.
            move_direction (numpy.array): direction along which the monomer
                will be positioned

def _add_monomer(self, monomer, mon_vector, move_direction):
        """
        extend the polymer molecule by adding a monomer along mon_vector direction

        Args:
            monomer (Molecule): monomer molecule
            mon_vector (numpy.array): monomer vector that points from head to tail.
            move_direction (numpy.array): direction along which the monomer
                will be positioned
        """
        translate_by = self.molecule.cart_coords[self.end] + \
                       self.link_distance * move_direction
        monomer.translate_sites(range(len(monomer)), translate_by)
        if not self.linear_chain:
            self._align_monomer(monomer, mon_vector, move_direction)
        # add monomer if there are no crossings
        does_cross = False
        for i, site in enumerate(monomer):
            try:
                self.molecule.append(site.specie, site.coords,
                                     properties=site.properties)
            except:
                does_cross = True
                polymer_length = len(self.molecule)
                self.molecule.remove_sites(
                    range(polymer_length - i, polymer_length))
                break
        if not does_cross:
            self.length += 1
            self.end += len(self.monomer)