Function to get ordered levels of output_type_ids for pmf or categorical output types

[elray1]

For the cdf output type as well as the pmf output type for targets with a target_type of ordinal, the order of the output_type_ids matters for validation, plotting and evaluation:

• For validation of cdf types, we need to ensure that the probabilities are non-decreasing across the category levels; this requires knowing the correct order of the category levels. See this issue.
• For plotting, we will probably want to arrange the categories in the right order
• For evaluation, some scores depend on getting the categories in the right order (e.g., this is required to calculate RPS for pmf output type)

To support those use cases, I propose adding a function here that extracts the output_type_id values that were specified in the tasks.json file, in the order they were specified there (so that we are taking the order they are given in the tasks.json file as a specification of the order of those levels.). This function probably would need to take a round_id and enough information to look up the category orders -- probably output_type and some set of task id variables?