Postmodeling: Reorganize Tutorial

We should consider restructuring the [postmodeling tutorial notebook](https://github.com/dssg/triage/blob/master/src/triage/component/postmodeling/contrast/postmodeling_tutorial.ipynb) to provide a more natural user flow, for instance:
- Moving the model group evaluation and performance metrics up front to potentially help choose a model to investigate in more detail.
- Adding a little more guidance on how to make use of the different postmodeling tools, what actions to consider taking based on the results of each analysis.

Open src/triage/component/postmodeling/contrast/postmodeling_tutorial.ipynb and review the current sequence of postmodeling analyses. Consider how model group evaluation and performance metrics could guide which model to investigate, then define the guidance users need for interpreting each tool and choosing follow-up actions. Done means the notebook presents a natural user flow and explains how to act on its results.

Written by the indexing model from the issue text.