FEA: Try out Scann

[garyfeng]

See Google's ScANN at https://github.com/google-research/google-research/blob/master/scann/docs/algorithms.md.

• Simple pip installation
• Apparently high performance according to benchmarks; not sure if we need that
• For <20k vector, recommending brute force; for >100k vectors, use score - search - rescore