Save `history_length` and `forecast_length` to disk for each modality, and use these in dataloader

[JackKelly]

When we use different sample frequencies and/or different durations for each modailty, then our ML code will need to know the history_length and forecast_length for each modailty, in order to work out what's in the past and what's in the future 🙂

[peterdudfield]

think this is now done

[JackKelly]

OK, cool, thanks for the update... Please do close this issue if you're sure 🙂