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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 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)