Return forecasts for easy processing with scoringutils

[sbfnk]

Is your feature request related to a problem? Please describe:
Forecast evaluation is an important part of broader workflows around forecasting. scoringutils is an R package for evaluating forecasts. It would be nice to integrate tightly with it by returning forecasts and data in a format that allows for easy processing there.

Describe the solution you'd like:
Functions like estimate_infections, simulate_infections, forecast_infections to return forecast in data that allows for calling of scoringutils::as_forecast() without any further processing - either directly or (perhaps more likely) through a function get_forecast()

[seabbs]

or a custom as.forecast method?

[jamesmbaazam]

or a custom as.forecast method?

I'm in favour of this since it'll live here instead of scoringutils (even though we have control over this since they're both developed by us).

[jamesmbaazam]

Conditional on #451.