Function group_by_assignments 

pub fn group_by_assignments(
    num_reps: usize,
    best_ids: &Array1<u32>,
) -> (Array1<u32>, Array1<u32>)

Groups data point indices by the representative they were assigned to, stored as two flat arrays instead of a list of lists. Example: best_ids = [1, 0, 1, 2], num_reps = 3.

groups (the intuitive shape): [[1], [0, 2], [3]]
offsets, data (what this returns): [0, 1, 3, 4], [1, 0, 2, 3]

groups[r] is data[offsets[r]..offsets[r+1]].