docs: def as_contours(self):
          """A dictionary of lists of contours keyed by byte_value"""
          contours = dict()
          for byte_value in self.__byte_values:
              if byte_value == 0:
                  continue
              mask = (self.__array == byte_value) * 255
              found_contours = find_contours(mask, 254, fully_connected='high') # a list of array
              contours[byte_value] = ContourSet(found_contours)
          return contours