docs: def count_below_mean(x):
      """
      Returns the number of values in x that are lower than the mean of x
  
      :param x: the time series to calculate the feature of
      :type x: numpy.ndarray
      :return: the value of this feature
      :return type: float
      """
      m = np.mean(x)
      return np.where(x < m)[0].size