def errors
      @errors ||= @time_series.zip(one_step_ahead_forecasts).map do |(observation, forecast)|
        forecast - observation if forecast && observation
      end
    end
def errors
      @errors ||= @time_series.zip(one_step_ahead_forecasts).map do |(observation, forecast)|
        forecast - observation if forecast && observation
      end
    end
Returns the errors between the observed values and the one step
 ahead forecasts.