docs: def _get_reference_band(self, data):
          """ Extract the NDWI band from time-series
  
          :param data: 4D array from which to compute the NDWI reference band
          :type data: numpy array
          :return: 3D array containing the NDWI reference band
          """
          nir = data[..., self.nir_idx].astype("float32")
          swir1 = data[..., self.swir1_idx].astype("float32")
          return (nir - swir1) / (nir + swir1)