kNN detector defaults

[mauicv]

Set the default aggregator for the kNN detector to the top-k aggregator with k set to a sensible fraction (e.g. 1/3) of the number of detectors. The intuition here is that being extremely outlying in a small number of ways shouldn't be cancelled out by being inlying in the majority of ways.

Note: this is part of the outlier detector effort and refers to work done in #677.