Add DNO aggregation routes

[braddf]

Detailed Description

Currently DNO aggregation is being handled by Quartz Solar in the browser, by requesting all GSP data and aggregating; could do this on the API side and offer DNO-level data explicitly, which would reduce some over-the-wire data/download time if GSP data not necessarily needed, and can also be cached.

Context

Quartz Solar could use this, and also general API users who'd like DNO aggregated data.

Possible Implementation

Create routes to mirror the GSP functions, such as:

• /v0/solar/GB/dno/forecast/all/
• /v0/solar/GB/dno/{dno_id}/forecast/
• /v0/solar/GB/dno/pvlive/all/
• /v0/solar/GB/dno/{dno_id}/pvlive

N.B. – GSP to DNO mappings are here for reference so JSON data can be copied into this repo when needed.
Also, may be self explanatory, but the aggregation logic currently implemented in the frontend is here if needed 👍