Build/document a way to tune nearest_neighbour settings

[wbecker]

At the moment it is hard to know what settings are best/effective (wrt early_stop, skipped_stop, max_time).

Would be good to:

1. post some guidelines about what values work well
2. write up a code snippet that demonstrates how to easily tune this, or
3. build it into the tool, perhaps on top of Classification use-case #16

[wbecker]

Sorry for spamming with all these issues @vmarkovtsev! Let me know if you'd rather I didn't, or communicated some other way!

[vmarkovtsev]

Hehe, no problem! I will review everything on Monday, thanks for your efforts.

[vmarkovtsev]

Choosing the right stop conditions is tricky. Some of them are defensive against insane outliers, some are enforcing heuristics. I certainly need to document them properly.