Add 4 hour horizon

[peterdudfield]

Add 4 hour forecast onto both national and GSPS graphs. This will show the forecast made 4 hours before target_time. This is useful for the control to see how the 4 hour forecast is doing.

Done: This needs work in the API to expose some new data. @peterdudfield needs to do this

Also need to define the colour of the line on the map. We are thinking some sort of blue.

Colour: could be 50% opacity yellow, or light blue

it should behind the yellow and black line

[peterdudfield]

API is now ready - https://api-dev.nowcasting.io/docs

/v0/GB/solar/gsp/forecast/latest/{gsp_id} has a 'forecast_horizon_minutes' input

[peterdudfield]

protoytype from @braddf is here