return the bottom grain (Structure) of the GB.

def bottom_grain(self):
        """
        return the bottom grain (Structure) of the GB.
        """
        bottom_sites = []
        for i, tag in enumerate(self.site_properties['grain_label']):
            if 'bottom' in tag:
                bottom_sites.append(self.sites[i])
        return Structure.from_sites(bottom_sites)