Create tactical demand forecasts

- the first notebook runs the tactical-forecasts command
- the second notebook describes the tactical demand forecasting process
  + demand aggregation on a per-pixel level
  + time series generation: horizontal, vertical, and real-time time series
  + STL decomposition into seasonal, trend, and residual components
  + choosing the most promising forecasting model
  + predicting demand with various models
- fix where to re-start the forecasting process after it was interrupted
- enable the heuristic for choosing the most promising model
  to also work for 7 training weeks