return the top grain (Structure) of the GB.

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