Add Lloyd aggregation with exact cluster centers
Add Lloyd aggregation with exact cluster centers

- Add a new `lloyd_cluster_exact()` that uses Floyd-Warshall to find the exact cluster centers and the "balanced" version of Bellman-Ford.

- Revert `lloyd_cluster()` to its original form, using the fast heuristic center-finder and plain Bellman-Ford.

- Rename `bellman_ford_adv()` to `bellman_ford_balanced()`.

- Fix `bellman_ford_balanced()` to ensure connectedness of the clusters.

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