Add GB electricity demand data

[JackKelly]

Detailed Description

Now that nowcasting dataset can handle different durations for different modalities, we could try adding electricity demand data to the inputs, and try predicting demand. (in the past, I used 8 days of historical demand data for each of the 6 DMDC points).

Possible Implementation

I think half hourly GB demand for each DMDC point is available from ELEXON. Maybe through Ayrton's Python wrapper?

This is maybe something to do in WP2