clean up description of groups of columns in model output

[elray1]

Looking at this page: https://hubverse.io/en/latest/user-guide/model-output.html#model-output-format

Screenshot of specific text block:

Comments:

• I believe the intent here was probably to nest points 3 through 5 as a sublist under point 2, but they ended up as items at the same outer list level.
• However, I'd suggest updating this as follows:

2. "Model output representation" (2 columns): consists of two columns specifying how the model outputs are represented. Both of these columns will be present in all model output data:
   1. output_type ...
   2. output_type_id ...
3. value contains the model's prediction.

The rationale here is that output_type and output_type_id specify how model outputs are represented, but value is the actual model output rather than metadata about the representation of that output.

(We've iterated on this text in a past PR, but I couldn't quickly find a link with meaningful discussion.)

[micokoch]

Hi Evan. I remember we talked about this, and I used the structure in the hubEnsembles manuscript. Anyway, I updated the structure as you suggested in PR #202.