feature: adding aggregations

[lorenzo-solcast]

Adding a module solcast.aggregations to query the API for live/forecast aggregations. It has no test so far as we are yet to expose unmetered aggregations.

[michaelf-solcast]

Is there an endpoint that lists the collections and aggregations available to the user? It can be a challenge with aggs to work out what collection_id and aggregation_id to use.

[lorenzo-solcast]

there is but is not a public API (yet). Suggest we don't do not add until it is exposed in the API docs.

[michaelf-solcast]

Also need to add to the list of available modules in index.md.