Computes the distance between the last two evaluations.

def _distance_last_evaluations(self):
        """
        Computes the distance between the last two evaluations.
        """
        if self.X.shape[0] < 2:
            # less than 2 evaluations
            return np.inf
        return np.sqrt(np.sum((self.X[-1, :] - self.X[-2, :]) ** 2))