Feedback: Return plevels when querying GSP endpoint

[JamieTaylor-TUOS]

When querying the GSP endpoint (/v0/solar/GB/gsp/{gsp_id}/forecast), optionally return the plevels as per national endpoint (/v0/solar/GB/national/forecast)

It's useful (less complex) to be able to access both GSP and national forecasts via a single endpoint. Atm if one does this using the GSP endpoint (setting gsp_id to 0 for national), it is not possible to retrieve the plevels (~error bars) associated with the national forecast.

The GSP forecasts do not (yet) compute plevels, so they would return an empty dict for the plevels key within forecastValues.

To avoid returning additional data unnecessarily and to avoid breaking any existing interfaces with the GSP endpoint, could add a new REST param to the GSP endpoint to tell the API to return the plevels: include_plevels?

Ultimately, it would be nice to get consistency between all endpoints in terms of REST params, e.g. so that calling:

/v0/solar/GB/national/forecast?include_metadata=false&start_datetime_utc=2023-10-06T00:30:00&end_datetime_utc=2023-10-06T00:30:00&creation_limit_utc=2023-10-05T07:00:00

yields an identical response to

/v0/solar/GB/gsp/0/forecast?include_metadata=false&start_datetime_utc=2023-10-06T00:30:00&end_datetime_utc=2023-10-06T00:30:00&creation_limit_utc=2023-10-05T07:00:00