ENH: cluster: reimplement the update-step of K-means in Cython
MAINT: cluster: remove the conditional import for _vq
MAINT: cluster: some cleanups in vq.pyx
Merge pull request #3712 from richardtsai/kmeans_update_rewrite

ENH: cluster: reimplement the update-step of K-means in Cython
BLD: special: link _ufuncs against LAPACK