Make `sample_to_quantile()` require a forecast object of class `forecast_sample`

[nikosbosse]

On the one hand, it's a data-wrangling function meant to make your life easier. On the other hand, we convert from a very specific format to another very specific format. If we decide to do that, then sample_to_quantile() should also output a validated forecast obejct

[seabbs]

I like the idea of locking hte input to be forecast_sample and outputting a forecast_quantile

[nikosbosse]

Okidokes, will do that then.