Extend National Electricity Demand forecast

[JackKelly]

National Grid ESO are interested in having an improved forecast for National Electricity Demand. And, because all British solar is on the distribution grid, solar generation looks like a reduction in 'demand' to ESO. So, it would be super-interesting to feed our PV nowcast into the national elec demand forecast model that Jack built for ESO in the first half of 2021 when he was consulting for ESO.

Some additional improvements to the Demand Forecast:

• Use full, gridded NWPs
• Instead of just a 'IS_HOLIDAY' feature, tell the model about different holidays, including holidays that only happen in one part of the UK. Might also need to over-sample holidays during training, because they occur so infrequently in the dataset.