Allow Images in stimulus templates

[rly]

Motivation

Fix #1637
Also add new test utility class that is more flexible than NWBH5IOMixin and which will replace that class eventually.

How to test the behavior?

Show how to reproduce the new behavior (can be a bug fix or a new feature)

Checklist

• Did you update CHANGELOG.md with your changes?
• Have you checked our Contributing document?
• Have you ensured the PR clearly describes the problem and the solution?
• Is your contribution compliant with our coding style? This can be checked running flake8 from the source directory.
• Have you checked to ensure that there aren't other open Pull Requests for the same change?
• Have you included the relevant issue number using "Fix #XXX" notation where XXX is the issue number? By including "Fix #XXX" you allow GitHub to close issue #XXX when the PR is merged.

[codecov]

Codecov Report

Merging #1638 (dababb1) into dev (4a38196) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##              dev    #1638   +/-   ##
=======================================
  Coverage   91.31%   91.32%           
=======================================
  Files          25       25           
  Lines        2534     2535    +1     
  Branches      481      481           
=======================================
+ Hits         2314     2315    +1     
  Misses        139      139           
  Partials       81       81           

Flag	Coverage Δ
integration	70.49% <100.00%> (+0.05%)	⬆️
unit	84.37% <100.00%> (+<0.01%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
src/pynwb/file.py	88.02% <100.00%> (+0.02%)	⬆️

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[bendichter]

So you can add Images but not Image directly? I'm OK with that I just wanted to confirm that this is what we want.

[rly]

Correct