Create internal R function to transform cdf model-output data to scoringutils-compatible format

[bsweger]

(formerly part of #12)

Create an internal data transformation function in hubEvals that will transform cdf model-output data into a format that can be successfully converted to a scoringutils forecast object: https://epiforecasts.io/scoringutils/dev/reference/as_forecast.html.

This is not a user-facing function. Instead, it will be invoked by the publicly-accessible function described in #11.

A good source of test data for this function is the forecast_outputs data object in the hubExamples repo (filtering on output_type == cdf)

https://github.com/Infectious-Disease-Modeling-Hubs/hubExamples/blob/main/data/forecast_outputs.rda

Definition of done:

• A new R function called transform_cdf_model_output
• Corresponding unit test(s)