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.
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.