docs: def ratio_value_number_to_time_series_length(x):
      """
      Returns a factor which is 1 if all values in the time series occur only once,
      and below one if this is not the case.
      In principle, it just returns
  
          # unique values / # values
  
      :param x: the time series to calculate the feature of
      :type x: numpy.ndarray
      :return: the value of this feature
      :return type: float
      """
      if not isinstance(x, (np.ndarray, pd.Series)):
          x = np.asarray(x)
      if x.size == 0:
          return np.nan
  
      return np.unique(x).size / x.size