Add basic CI workflow

[brownbaerchen]

This PR adds basic infrastructure for CI

• tests directory where all the tests will go. Currently there are only two dummy tests there
• Github workflow that installs the code and then runs the tests

The problem is: The installation already fails. See the workflow logs.

So this needs a lot of work, but is intended as a starting point for some CI. Once there are tests, we should add a coverage report as well.

[brownbaerchen]

Change to pyproject.toml with dependencies: numpy, matplotlib, astropy, sklearn, itertools, scipy, tqdm

[brownbaerchen]

I had to shuffle around the files to make scimes importable as a package. There is also a bit of a mess with the license, which needs sorting out. See the workflow run on my fork for the current state.
Note that the jobs are now failing because they run the failing dummy test, as intended. Once you have seen how a failing action looks, we can remove both dummy tests.

The further workflow I would suggest is

• you have a review of this PR
• once you're happy I remove the dummy tests
• we merge with only the installation tests as fast as possible

Because the files are shuffled around, a quick merge is good to not escalate merge conflicts

Then, you can gradually add tests, maybe a coverage report, releases and so on. But I do think that having the package be installable is quite important and should precede any further developments. This will also make it easier to figure out where to put @LarsvE568`s stuff.