def ndwi(self):
data = self._read(self[self._ndwi_bands,...]).astype(np.float32)<EOL>return (data[<NUM_LIT:1>,:,:] - data[<NUM_LIT:0>,:,:]) / (data[<NUM_LIT:0>,:,:] + data[<NUM_LIT:1>,:,:])<EOL>
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