filter by forecast name in GetLatestForecasts

[peterdudfield]

When working on the API we currently get the latest forecast, it would be good to filter by forecast_name.

When trying to load national/forecast, when need to load get_latest_forecasts and we there's an option to get it for a certain model name. I think added a forecast_name to filter by would also speed up the query.

This will be need for providing forecast metadata in the API