Add Box3D, flake8 & cleanups

[jstriebel]

This PR refactors the existing code to be much more readable.
It introduces

• Box3D, which has math operators that simplify the code of the neuroglancer backend,
• from_json can now also load Union tuples (useful for Optional),
• flake8, that finds unused imports,
• isort, which sorts the imports, and
• some minor cleanups, e.g.
  • remove the precomputed:// prefix from the source links
  • do not use pre-releases anymore (except for black)

[fm3]

Cool stuff! That will help a lot with readability :) Especially the Box3D.
Just to make sure, I assume removing tools/sample-datasets/neuroglancer.json was intentional?

[fm3]

👍

[jstriebel]

Yes, removing tools/sample-datasets/neuroglancer.json was intentional, it's not necessary since we have data/datasets.json.