Helper function for calculation of split-half cross-validation model
        error and signal reliability.

def _xval_model_error(self, fit_spectra, reject_outliers, fit_lb, fit_ub,
                           fitter, func):
        """
        Helper function for calculation of split-half cross-validation model
        error and signal reliability.

        """
        set1 = fit_spectra[::2]
        set2 = fit_spectra[1::2]
        errs = []
        signal_select = [] 
        models = []
        signals = []
        for this_set in [set1, set2]:
            choose_transients, model, signal, params, this_idx =\
                self._fit_helper(this_set, reject_outliers,
                                 fit_lb, fit_ub, fitter)
            models.append(np.nanmean(model[choose_transients], 0))
            signals.append(np.nanmean(signal[choose_transients], 0))

            signal_select.append(signal[choose_transients])
                
        #Cross-validation error estimation:
        model_err = np.mean([ut.rmse(models[0], signals[1]),
                              ut.rmse(models[1], signals[0])])
        # Also for the signal:
        signal_err = ut.rmse(np.nanmean(signal_select[0], 0),
                             np.nanmean(signal_select[1], 0))
        # Based on the errors, choose a function. Also report errors:
        return model_err, signal_err