Generates a new `Assembly` containing only the β-strands.

        Notes
        -----
        Metadata is not currently preserved from the parent object.

        Returns
        -------
        strand_assembly : ampal.Protein
            `Assembly` containing only the β-strands of the original `Assembly`.

def strands(self):
        """Generates a new `Assembly` containing only the β-strands.

        Notes
        -----
        Metadata is not currently preserved from the parent object.

        Returns
        -------
        strand_assembly : ampal.Protein
            `Assembly` containing only the β-strands of the original `Assembly`.
        """
        strand_molecules = list(itertools.chain(
            *[p.strands._molecules for p in self._molecules if hasattr(p, 'strands')]))
        strand_assembly = Assembly(
            molecules=strand_molecules, assembly_id=self.id)
        return strand_assembly