Improve code quality

[JulienPeloton]

In this PR, we check the code quality with a linter (flake8). Here is the command used:

black -l 86 s4cmb/*
flake8 s4cmb/*.py --count --show-source --statistics --max-line-length 86 --doctests --ignore=E741,W605

Missing:

• Add the command above in the GH actions checks (a different workflow)

[coveralls]

Coverage increased (+0.01%) to 86.949% when pulling 35a7263 on issues/51/codequality into ec064cd on master.