Faster building / batch-insert using compression

[thomasahle]

Currently IVF.fit(...) uses brute force nearest neighbours to find which clusters to insert the points into.
Instead we could use the same PQ.top(...) method that we use to do queries to find the relevant cluster centers faster.
However, currently PQ.top(...) doesn't support batch queries, which means this likely wouldn't be faster than brute force.

Hence the task: Find a way to do fast batch queries with QuickADC and add it to FastPQ.