Split `as_forecast()` into separate functions for the different forecast types

[nikosbosse]

As mentioned by @seabbs somewhere, it would be a good idea to split up as_forecast() into dedicated functions. The rationale is that this

• makes everything cleaner in terms of code (since as_forecast() doesn't have to automagically infer the forecast type)
• forces users to form a clear intent, making the overall path clearer and avoiding errors along the way