Tox managment, Black formatting

[shanemcq18]

No API changes here, just some changes to the meta.

• Switched to tox for state-of-the-art dependency and testing management
• Made black formatting a requirement. Format the code with make format or tox -e format, check style with make style or tox -e style.
• Formatted the entire package with black (not every file was changed, just those that haven't been touched in a while).
• Updated contributor instructions for tox and black.
• Reduced the number of GitHub Actions tests; should have 12 unit test actions (4 python versions, 3 platforms), and a single test for code style.
• Removing the docs build action for now, need to figure out how to download data files for the notebooks to execute correctly (see Data download script #60).