Number of these atoms occupying a specific site type.

        Args:
            site_label (Str): Label for the site type being considered.

        Returns:
            (Int): Number of atoms occupying sites of type `site_label`.

def occupations( self, site_label ):
        """
        Number of these atoms occupying a specific site type.

        Args:
            site_label (Str): Label for the site type being considered.

        Returns:
            (Int): Number of atoms occupying sites of type `site_label`.
        """
        return sum( atom.site.label == site_label for atom in self.atoms )