Free speedup of flats processing
For now, each passes functions call a bunch of steps with the whole flats list. This means we iterate over all the flats at each substep.
See https://framagit.org/phyks/Flatisfy/blob/master/flatisfy/filters/init.py#L163-182.
Flats are interdependent so this could be done with a single for flat in flats_list
iteration at the pass level (e.g. https://framagit.org/phyks/Flatisfy/blob/master/flatisfy/filters/init.py#L162).
This single loop could even be passed down to joblib
for parallel processing: https://pythonhosted.org/joblib/parallel.html.