Set parameters in cell.in for a BSE computation
        :param nv_bse: number of valence bands
        :param nc_bse: number of conduction bands
        :param n_excitations: number of excitations
        :param nit_bse: number of iterations

def set_BSE_options(self, n_excitations=10, nit_bse=200):
        """
        Set parameters in cell.in for a BSE computation
        :param nv_bse: number of valence bands
        :param nc_bse: number of conduction bands
        :param n_excitations: number of excitations
        :param nit_bse: number of iterations
        """

        self.BSE_TDDFT_options.update(npsi_bse=n_excitations, nit_bse=nit_bse)