Extract the max-ratio band from time-series

            The max-ratio is defined as max(NIR,SWIR1)/BLUE

            :param data: 4D array from which to compute the max-ratio reference band
            :type data: numpy array
            :return: 3D array containing the max-ratio reference band

def _get_reference_band(self, data):
        """ Extract the max-ratio band from time-series

            The max-ratio is defined as max(NIR,SWIR1)/BLUE

            :param data: 4D array from which to compute the max-ratio reference band
            :type data: numpy array
            :return: 3D array containing the max-ratio reference band
        """
        blue = data[..., self.blue_idx].astype("float32")
        nir = data[..., self.nir_idx].astype("float32")
        swir1 = data[..., self.swir1_idx].astype("float32")
        return np.nanmax(np.array([nir, swir1]), axis=0) / blue