Switch nbval comments to tags

[jennyfothergill]

This will clean up the way our tutorials look for users--tags are hidden--but would retain the function of skipping cells we know will fail in CI.

More info https://nbviewer.jupyter.org/github/computationalmodelling/nbval/blob/0.9.1/docs/source/index.ipynb#Using-tags-instead-of-comments

[mikemhenry]

Another option would be to do this:

try:
    #  some code that throws an Error
except Error as e:
    print(e)

[jennyfothergill]

Maybe even using this nbval comment (which verifies that the stored exception is thrown) as a tag would be ideal https://nbviewer.jupyter.org/github/computationalmodelling/nbval/blob/master/docs/source/index.ipynb#Checking-exceptions

[mikemhenry]

I can confirm using the tag works:
https://github.com/glotzerlab/fresnel-examples/pull/9/files#diff-cb920c31dca3318e07a8eb2d05a8e03aR303-R307

Which has the added bonus that the notebook will keep running if you hit "run all cells" instead of stopping when an exception is thrown