text: Title: time series forecasting: window features
  Body: The transformer should create computations over windows of past values of the features, and populate them at time t, t being the time of the forecast.
  
  It uses pandas rolling, outputs several comptutations, mean, max, std, etc, and pandas shift to move the computations to the right row.
  
  ```
  tmp = (data[variables]
         .rolling(window='3H').mean()  # Average the last 3 hr values.
         .shift(freq='1H')  # Move the average 1 step forward
         )
  
  # Rename the columns
  tmp.columns = [v + '_window' for v in variables]
  
  data = data.merge(tmp, left_index=True, right_index=True, how='left')
  ```
  
  