Returns the site properties as a dict of sequences. E.g.,
        {"magmom": (5,-5), "charge": (-4,4)}.

def site_properties(self):
        """
        Returns the site properties as a dict of sequences. E.g.,
        {"magmom": (5,-5), "charge": (-4,4)}.
        """
        props = {}
        prop_keys = set()
        for site in self:
            prop_keys.update(site.properties.keys())

        for k in prop_keys:
            props[k] = [site.properties.get(k, None) for site in self]
        return props