Add spin states to a structure by site.

        Args:
            spins (list): List of spins
                E.g., [+5, -5, 0, 0]

def add_spin_by_site(self, spins):
        """
        Add spin states to a structure by site.

        Args:
            spins (list): List of spins
                E.g., [+5, -5, 0, 0]
        """
        if len(spins) != len(self.sites):
            raise ValueError("Spin of all sites must be "
                             "specified in the dictionary.")

        for site, spin in zip(self.sites, spins):
            new_sp = {}
            for sp, occu in site.species.items():
                sym = sp.symbol
                oxi_state = getattr(sp, "oxi_state", None)
                new_sp[Specie(sym, oxidation_state=oxi_state,
                              properties={'spin': spin})] = occu
            site.species = new_sp