Args:
            names (tuple[str]): the names ('train' or 'test') of the datasets

        Returns:
            An array of three values as mean of each channel, for all images in the given datasets.

def get_per_channel_mean(self, names=('train', 'test')):
        """
        Args:
            names (tuple[str]): the names ('train' or 'test') of the datasets

        Returns:
            An array of three values as mean of each channel, for all images in the given datasets.
        """
        mean = self.get_per_pixel_mean(names)
        return np.mean(mean, axis=(0, 1))