docs: def get_superpxl_intensities(im, suppxls):
      """Calculates mean intensities of pixels in superpixels
      inputs:
          im ... grayscale image, ndarray [MxN]
          suppxls ... image with suppxls labels, ndarray -same shape as im
      outputs:
          suppxl_intens ... vector with suppxls mean intensities
      """
      n_suppxl = np.int(suppxls.max() + 1)
      suppxl_intens = np.zeros(n_suppxl)
  
      for i in range(n_suppxl):
          sup = suppxls == i
          vals = im[np.nonzero(sup)]
          try:
              suppxl_intens[i] = np.mean(vals)
          except:
              suppxl_intens[i] = -1
  
      return suppxl_intens