Make use of TODO comments

The idea is that we can leave TODO and it will show as warning in the pipeline because it can be detected by pylint.
Right now, there are a lot of TODO left by Nicolas, some possible impovements or sometime just kind of explanations (in functions.py ).
I'd like to clean those, convert actual features suggestions to Issues (or not) then we can remove the "fixme" pylint exception, and use TODO tags as they were meant in the first place.