write specific functions to translate hubverse model output to different scoringutils prediction type dfs

[nickreich]

These helper functions would be called by a more general translation function. Output types needed to support

• [ ] quantile
• [ ] sample
• [ ] cdf
• [ ] pmf --> something that doesn't exist yet in scoringutils (see issue)
• [ ] mean/median (hubverse) --> point (scoringutils)

migrated from https://github.com/epiforecasts/scoringutils/issues/605

[bsweger]

We split this into a single issue for each model_output type, so closing this one.

See instead:

• 22 (quantile)

• 23 (cdf)

• 24 (pmf stub)

• 25 (mean/median)

• 26 (sample)