Computes the Expected Improvement per unit of cost

def _compute_acq(self, x):
        """
        Computes the Expected Improvement per unit of cost
        """
        m, s = self.model.predict(x)
        fmin = self.model.get_fmin()
        phi, Phi, u = get_quantiles(self.jitter, fmin, m, s)
        f_acqu = s * (u * Phi + phi)
        return f_acqu