Add `discard_spikes`/`clean_spikes` step in a curation v3

Right now, the curation has:

• removed: units to remove
• merges: unit groups to merge
• splits: unit to split into 2 or multiple sub-units

One another very common use case is to "clean" units. For example, if you have a nice and clean unit with a very nasty noisy event it's easy to remove (most) of those noisy spikes. Currently, to achieve this, one would need a 2 step curation: the first one to perform the split, the second to remove the "noisy" splitted unit.

This is to propose another "step" in the curation model (and make a version 3) which does both of the steps in one go!
It should be very easy on the implementation side: the "discard_spikes" could be run as the first step, and internally perform splits whose units to remove would just be added to the removed units.

Start by locating the curation v2 model and the existing split and removed-unit handling, since the issue names no files or tests. Trace how a split is represented and how removed units are collected. Done means curation v3 supports a discard_spikes or clean_spikes step that combines those operations while preserving the existing removed, merges, and splits behavior.