better options for adjusting start_date

In the process of debugging why the pipeline was doing poorly on a specific day, we needed to adjust the start date. Eventually we used a pair of browsers, one in `slide_forecaster` and one in the actual forecast, with manual start date adjustment. This is jank.

In general, `slide_training` and `n_training_pad` are pretty awkward to use as-is, and should probably get refactored.