code: def ndvi(self, **kwargs):
          
          data = self._read(self[self._ndvi_bands,...]).astype(np.float32)
          return (data[0,:,:] - data[1,:,:]) / (data[0,:,:] + data[1,:,:])
docs: Calculates Normalized Difference Vegetation Index using NIR and Red of an image.
  
          Returns: numpy array with ndvi values