Calculate the quantization error.

        Find the the minimum euclidean distance between the units and
        some input.

        Parameters
        ----------
        X : numpy array.
            The input data.
        batch_size : int
            The batch size to use for processing.

        Returns
        -------
        error : numpy array
            The error for each data point.

def quantization_error(self, X, batch_size=1):
        """
        Calculate the quantization error.

        Find the the minimum euclidean distance between the units and
        some input.

        Parameters
        ----------
        X : numpy array.
            The input data.
        batch_size : int
            The batch size to use for processing.

        Returns
        -------
        error : numpy array
            The error for each data point.

        """
        dist = self.transform(X, batch_size)
        res = dist.__getattribute__(self.valfunc)(1)

        return res