ENH refactor squared-norms computation to extmath

k-means can now use the memory-efficient dense implementation from
metrics.pairwise, while pairwise can use the fast sparse implementation
from k-means.
ENH use row_norms in KDE code

There may be other places where it is useful.
COSMIT remove dead code in k-means

Private functions _k_init and _kmeans_single were doing too much work.

Should improve the relative coverage.
Merge branch 'refactor-squared-norms'