forecast_len: 0 is confusing

[sethmcg]

The model config files (e.g., example.yml) use array index offsets for length parameters. E.g., for single-step training, the model returns indexes i to i+0, so the yaml file says forecast_len: 0.

This is confusing for end-users. If the model is predicting one step ahead and returning a single timestep, the length of the forecast is 1 timestep, so the forecast_len parameter should be 1, not 0.