code: def xypix_to_ipix(self, xypix, colwise=False):
          
          return np.ravel_multi_index(xypix, self.npix,
                                      order='F' if colwise else 'C',
                                      mode='raise')
docs: Return the flattened pixel indices from an array multi-dimensional
          pixel indices.
  
          Parameters
          ----------
          xypix : list
              List of pixel indices in the order (LON,LAT,ENERGY).
  
          colwise : bool
              Use column-wise pixel indexing.