docs: def pca_to_mapping(pca,**extra_props):
      """
      A helper to return a mapping of a PCA result set suitable for
      reconstructing a planar error surface in other software packages
  
      kwargs: method (defaults to sampling axes)
      """
      from .axes import sampling_axes
      method = extra_props.pop('method',sampling_axes)
      return dict(
          axes=pca.axes.tolist(),
          covariance=method(pca).tolist(),
          **extra_props)