code: def ipix_to_xypix(self, ipix, colwise=False):
          
          return np.unravel_index(ipix, self.npix,
                                  order='F' if colwise else 'C')
docs: Return array multi-dimensional pixel indices from flattened index.
  
          Parameters
          ----------
          colwise : bool
              Use column-wise pixel indexing.