docs: def get_inert_ratio_prnc(cont):
      """Compute principal inertia ratio of a contour
  
      The principal inertia ratio is rotation-invariant, which
      makes it applicable to reservoir measurements where e.g.
      cells are not aligned with the channel.
  
      Parameters
      ----------
      cont: ndarray or list of ndarrays of shape (N,2)
          A 2D array that holds the contour of an event (in pixels)
          e.g. obtained using `mm.contour` where  `mm` is an instance
          of `RTDCBase`. The first and second columns of `cont`
          correspond to the x- and y-coordinates of the contour.
  
      Returns
      -------
      inert_ratio_prnc: float or ndarray of size N
          The principal inertia ratio of the contour
      """
      if isinstance(cont, np.ndarray):
          # If cont is an array, it is not a list of contours,
          # because contours can have different lengths.
          cont = [cont]
          ret_list = False
      else:
          ret_list = True
  
      length = len(cont)
  
      inert_ratio_prnc = np.zeros(length, dtype=float) * np.nan
  
      for ii in range(length):
          moments = cont_moments_cv(cont[ii])
          if moments is not None:
              # orientation of the contour
              orient = 0.5 * np.arctan2(2 * moments['mu11'],
                                        moments['mu02'] - moments['mu20'])
              # require floating point array (only copy if necessary)
              cc = np.array(cont[ii], dtype=float, copy=False)
              # rotate contour
              rho = np.sqrt(cc[:, 0]**2 + cc[:, 1]**2)
              phi = np.arctan2(cc[:, 1], cc[:, 0]) + orient + np.pi / 2
              cc[:, 0] = rho * np.cos(phi)
              cc[:, 1] = rho * np.sin(phi)
              # compute inertia ratio of rotated contour
              mprnc = cont_moments_cv(cc)
              inert_ratio_prnc[ii] = np.sqrt(mprnc["mu20"] / mprnc["mu02"])
  
      if not ret_list:
          inert_ratio_prnc = inert_ratio_prnc[0]
  
      return inert_ratio_prnc