Draft: ci: adding static analysis and technical debt reduction

This merge request is linked to #229 (closed).

I propose to add a linter job to the Gitlab CI in order to get regularly metrics about the current technical debt in DL. As a consequence, I fixed a lot of warnings raised by pep8 and pylint.

I let you check everything is ok. To the moment, I removed the changes that warned you, about method overriding with different parameters. I also excluded this from prospector’s analysis. One step at a time!

There were almost 400 warnings at the beginning, 180 remain.

Tell me if you want me to merge all the “refactor” commits into one (which I do not recommand to simplify any further bisect).