faster/better tree building algorithm (see #64)

[erikbern]

I haven't validated this with any real dataset, need to do that

[erikbern]

I want to run this on a real dataset to see if it helps.

I'm also a bit concern what happens in degenerate cases such as i==j but I think it will work out fine

[erikbern]

Before

Ran 21 tests in 115.291s

After

Ran 21 tests in 23.230s

[erikbern]

See accuracy test that I just added: f5c1f58

The improvements are pretty spectacular:

Before

             angular   25 accuracy: 34.64%
             angular   50 accuracy: 20.44%
             angular  100 accuracy: 12.90%
             angular  200 accuracy:  7.99%

           euclidean   25 accuracy: 33.71%
           euclidean   50 accuracy: 21.04%
           euclidean  100 accuracy: 13.37%
           euclidean  200 accuracy:  9.31%

After

             angular   25 accuracy: 46.80%
             angular   50 accuracy: 31.00%
             angular  100 accuracy: 24.50%
             angular  200 accuracy: 15.18%

           euclidean   25 accuracy: 47.34%
           euclidean   50 accuracy: 33.04%
           euclidean  100 accuracy: 25.10%
           euclidean  200 accuracy: 17.99%

The runtime of the tests also dropped from about 6000s to 1500s meaning a speedup of 4x.

Pretty cool for a code review that's mostly red!

[a1k0n]

what if i == j?

[a1k0n]

That is super awesome, though.