fix: increase ckmeans performance

[marcelgerber]

Fixes #520.

I don't know what I'm doing, but this way around it's closer to what the reference implementation does, and the tests still pass 🙃

And it's way faster: Results using @breck7's test code from #520 (comment):

Before (without this fix):

kSize,nSize,time
5,1000,22
5,2000,33
5,10000,180
5,20000,820
5,100000,37749
5,200000,201173

After (with this fix):

kSize,nSize,time
5,1000,15
5,2000,29
5,10000,32
5,20000,52
5,100000,125
5,200000,324

cc @schnerd for review if you find the time, and this may be something you want to fix in your own ckmeans packages, too.

[tmcw]

Wow, yep, looks like a typo that just survived because it was over-checking rather than under!

[marcelgerber]

Thanks for the quick merge and release, very much appreciated!

[yyx990805]

who else got here from https://www.youtube.com/watch?v=KRD5jiYUOhk ? :)