Introduce pre-commit hooks

[jlenain]

This PR introduces pre-commit hooks to our repository.

Following discussions with @sona-patel and #49 , in such a way, any new commit to the code will automatically:

• be linted using black and flake8, including for commits on Jupyter notebooks,
• see the different imports sorted using isort, following @kosack 's recommendations.

provided that developers enable pre-commits with (see https://pre-commit.com/#quick-start):

pre-commit install

TODO DONE

• A very nice suggestion by @HealthyPear in Add trigger patches ctapipe_io_nectarcam#40 (comment) concerns adding a pre-commit hook to automatically convert Jupyter notebooks using jupytext (see e.g. https://jupytext.readthedocs.io/en/latest/using-pre-commit.html). Will do in a following commit in this PR.

[jlenain]

@sona-patel , with such hooks, it is possible to lint the whole repository (see https://pre-commit.com/#4-optional-run-against-all-the-files) with:

pre-commit run --all-files

[codecov]

Codecov Report

Patch coverage has no change and project coverage change: +2.69% 🎉

Comparison is base (cdded69) 0.38% compared to head (e830503) 3.08%.
Report is 4 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff            @@
##           master     #71      +/-   ##
=========================================
+ Coverage    0.38%   3.08%   +2.69%     
=========================================
  Files          30      36       +6     
  Lines        3404    3571     +167     
=========================================
+ Hits           13     110      +97     
- Misses       3391    3461      +70     

see 7 files with indirect coverage changes

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