pub type ByPhasePartition<'a, T> = (BTreeMap<Phase, Vec<SampleTriple<'a, T>>>, Vec<SampleTriple<'a, T>>);Expand description
Return shape of SeriesField::by_phase: a BTreeMap keyed by
Phase carrying the samples whose source row had a stamped
step_index, plus a separate none_bucket for unstamped /
fixture samples.