`prepare_ml_data.py` should save separate logs to `OutputData.filepath/<SplitName>/<DataSource>`

[JackKelly]

Detailed Description

Save log to OutputData.filepath so that, in the future, it's possible to diagnose issues with the saved batches.

Also send log to the console.

[peterdudfield]

This would be handy

[JackKelly]

Now that each DataSource runs separately, it might make sense to save separate logs to OutputData.filepath/<SplitName>/<DataSource>

That will make it easier to diagnose errors specific to a data source.

(Perhaps also save a unified log to OutputData.filepath)

[JackKelly]

I'm going to work on this now because it'll be useful to help diagnose issues with nowcasting_dataset...