Args:
            x (float or np.ndarray): must have the same shape.

def feed(self, x):
        """
        Args:
            x (float or np.ndarray): must have the same shape.
        """
        self._n += 1
        delta = x - self._mean
        self._mean += delta * (1.0 / self._n)
        delta2 = x - self._mean
        self._M2 += delta * delta2