Computes the Expected Improvement and its derivative (has a very easy derivative!)

def _compute_acq_withGradients(self, x):
        """
        Computes the Expected Improvement and its derivative (has a very easy derivative!)
        """
        fmin = self.model.get_fmin()
        m, s, dmdx, dsdx = self.model.predict_withGradients(x)
        phi, Phi, u = get_quantiles(self.jitter, fmin, m, s)
        f_acqu = s * (u * Phi + phi)
        df_acqu = dsdx * phi - Phi * dmdx
        return f_acqu, df_acqu