Fix class-based cluster representative sampling

[atroyn]

#87 introduces the core algorithms, but representative sampling using class labels is currently broken.

This is probably because hnswlib filtering doesn't let us get a connected graph, and so ANN search fails. We should understand how / when this fails. There are possibly easy fixes, i.e. generating a separate index per dataset per class, or increasing the edge factor, but the real solution is to understand how to use hnsw properly in our context.

[jeffchuber]

Closing this.