write function that translates from hubverse format to scoringutils-ready format

[nickreich]

Create a user-accessible function to handle transforming model-output data to a scoringutil forecast object: https://epiforecasts.io/scoringutils/dev/reference/as_forecast.html

Essentially, the goal of this function is to ensure that model output data is converted to the corresponding scoringutil format by calling the appropriate internal transformation function (these functions are defined in #22, #23, #24, #25, #26).

For example, model output data of output_type median would be passed to the internal hubEvals transform_point_model_output function.

This function should be exported for a user who wants more direct, custom access to scoringutils functionality, but also will be called in hubEvals::score_mdl_output() (#13)

migrated from epiforecasts/scoringutils#605

Definition of done:

• Create function
• Unit test(s)
• Update documentation (vignettes, news, etc.)

[nickreich]

To clarify what distinguishes this issue from #12, I think the idea is that this function would be exported and accessible by users where the functions specified in #12 would not be.