Surprising behavior for `get_clusters` with the `first-k` method.

[bpkent]

When retrieving clusters with the first-k method, there is some surprising behavior for edge cases.

• If there are more root nodes than k, all roots are returned as clusters, so there are more than k clusters.
• If there are fewer than 'k', each leaf is a cluster, but this still leaves fewer than 'k' clusters.

The solution is probably to simply warn the user when the number of clusters returned isn't exactly 'k'.