code: def ndwi(self):
          
          data = self._read(self[self._ndwi_bands,...]).astype(np.float32)
          return (data[1,:,:] - data[0,:,:]) / (data[0,:,:] + data[1,:,:])
docs: Calculates Normalized Difference Water Index using Coastal and NIR2 bands for WV02, WV03.
          For Landsat8 and sentinel2 calculated by using Green and NIR bands.
  
          Returns: numpy array of ndwi values