Add extract_frames utility to help visualize test control data

[kjlubick]

Overview: What does this pull request change?

This adds a small Python script to extract the contents of a .npz file created by _ControlDataWriter as individual PNG frames.

Motivation and Explanation: Why and how do your changes improve the library?

This will make it easier for devs to make sure their graphical unit tests are drawing what they expect. I personally had a hard time only using --show_diffs to determine if my test was doing what I wanted.

Links to added or changed documentation pages

contributing/testing.html

Reviewer Checklist

• [ ] The PR title is descriptive enough for the changelog, and the PR is labeled correctly
• [ ] If applicable: newly added non-private functions and classes have a docstring including a short summary and a PARAMETERS section
• [ ] If applicable: newly added functions and classes are tested

[kjlubick]

Looks like build the docs timed out after ~20 minutes?

[kjlubick]

Friendly bump