Tests for Jupyter notebook tutorials

[dataspider]

Goal: Run cell-by-cell output comparisons against pre-run notebooks as part of our GitHub-CI unit-test suite.

Background/resources:

• XGI workflow example (uses pytest and conversion to scripts)
• Our test stub (uses unittest; currently subprocess + file path issues when running as part of suite in GitHub actions)
• testbook (dev status unclear)
• nbval
• nbcelltests