Adds a `Polymer` to the `Assembly`.

        Raises
        ------
        TypeError
            Raised if other is any type other than `Polymer`.

def append(self, item):
        """Adds a `Polymer` to the `Assembly`.

        Raises
        ------
        TypeError
            Raised if other is any type other than `Polymer`.
        """
        if isinstance(item, Polymer):
            self._molecules.append(item)
        else:
            raise TypeError(
                'Only Polymer objects can be appended to an Assembly.')
        return