Address some testing and packaging issues

[gipert]

Lots of new features: here's a short summary of the changes.

• Enable GitHub Dependabot to keep GitHub actions automatically up to date. There will be a bot opening a pull request once in a while (I expect it to be very quiet) to update the GitHub action workflow file.
• Add https://codecov.io configuration file (this will enable the corresponding bot) and upload coverage automatically with GitHub actions. For every pull request the Codecov bot will post a summary of the test coverage status and tell us whether the proposed changes are sufficiently tested. Let's see how noisy this is: we can temporary disable the bot while in this heavy-development phase
• Enable the https://pre-commit.ci bot to automatically perform style checks through pre-commit: there will be now an additional check run for every commit ensuring that coding conventions are respected. The pre-commit.ci bot will automatically fix (wherever possible) style-related issues in every PR, by adding a commit. The list of checks to be performed are specified through the .pre-commit-config.yaml configuration file. At the moment I just enabled some basic (but extremely useful!) checks: trailing whitespace or newlines, dead symlinks, large files... Later we can also enable some python linters (like black or flake8) or static type checkers (mypy): this will be also extremely useful but requires a massive re-format of the existing code – we can do it once the refactoring is over
• Update developer docs

See also #241 and #242.

[codecov-commenter]

Codecov Report

❗ No coverage uploaded for pull request base (refactor@4935723). Click here to learn what that means.
The diff coverage is n/a.

@@             Coverage Diff             @@
##             refactor     #255   +/-   ##
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  Coverage            ?   21.10%           
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  Files               ?       20           
  Lines               ?     2421           
  Branches            ?        0           
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  Hits                ?      511           
  Misses              ?     1910           
  Partials            ?        0           

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[gipert]

@jasondet, @iguinn: can check whether this looks fine to you? The changes are 90% due to pre-commit hooks removing trailing whitespace/newlines, checking spelling and sorting python imports – they should be innocuous. I did not enable hooks that apply formatting rules yet, we may do that once the refactoring is over and we'll maybe have more tests.

The very cool thing about these hooks is that they will be run for each PR and will keep the code automatically formatted.

@iguinn: I configured a pre-commit hook that strips output & metadata field from Jupyter notebooks – should be a better way to handle the issue. I will cleanup setup.py in a future PR.

[jasondet]

It looks good to me. I'm not going to go through and check the "viewed" box for all 89 files though ;)

I just merged the previous PR, can you fix the new conflicts?

[gipert]

Can this be merged? Objections?