Moved atlas_gen to separate package

[vigji]

In this PR:

• I moved the atlas_gen module to be a separate package; this in principle should make sure there's no confusion on what should be imported in the main package that needs to be tested;
• I removed 2 of the tests that could not be parallelelized;
• I removed the installation of the dev tools for the tests.

[vigji]

Final test structure:

• Linux (one version only): py3.6, 3.7, 3.8;
• Windows: py3.7 only;
• Separate test for linting, so black/flake8 errors are lumped together in a single place.

Tests are significantly faster now, although Windows still take some more time to setup. Maybe we could temporarily disable that together with macOS? I left all the code in the travis folder though, so put them back in the future will be straightforward.

I also added a separate requirements.txt file for easy deps parsing.

I think this should be mergeable now! @adamltyson ?

[adamltyson]

I won't have time to review this properly until tomorrow, but it looks ok.

I'm keen to keep as much testing as we can though, because I've come across python version/OS interactions in the past, when wheels are only built for specific versions etc.

although Windows still take some more time to setup. Maybe we could temporarily disable that together with macOS?

I think we should absolutely keep the Windows test, and I would like to bring the macOS test back if we can.

Ideally we would also test every combination, but as this takes a long time, maybe we can keep it for tagged commits (i.e. to prevent deployment if there is an issue)?

Sorry to be a pain, but I think the tests taking a bit longer isn't much of an issue if we make sure everything passes locally (and maybe be selective with the commits we push?). I'm also wary that I will be maintaining quite a lot of software that will likely depend on this package, so I'm keen to test as much as is practical.

[vigji]

Ok, I added back the macOS test.

True that testing locally should take care of most problems. Still, maybe all the matrix is an overkill for all commits. If you know a way of trigger the job will all OSs and all versions only on marked commits that would be great!

[vigji]

I will marge for now as I don't see potential issues, but let me know if something's not ok. I am happy to reintroduce the grid testing if it can restricted to tags in the future!

[adamltyson]

Cool, I've added this functionality in #30.