BUG: change ddof default to 1 in mstats.sem, consistent with stats.sem.

See discussion on https://github.com/scipy/scipy/pull/3458 for details.
BUG: cluster.vq.whiten returns nan or inf when std==0

When the standard deviation of a column of the observations is zero,
vq.whiten should raise a warning and set the zero-std to 1.0 to avoid
nan/inf which may cause the kmeans functions work incorrectly.
Fixes gh-3499.
Merge pull request #3639 from rgommers/sem-ddof

BUG: change ddof default to 1 in mstats.sem, consistent with stats.sem.
MAINT: stats: remove superfluous alias in mstats_basic.py
Merge pull request #3504 from richardtsai/vq_whiten_zero_std

BUG: cluster.vq.whiten returns nan or inf when std==0
Merge pull request #3644 from WarrenWeckesser/maint-mstats

MAINT: stats: remove superfluous alias in mstats_basic.py
ENH: Added support for date attributes to io.arff.arffread

Requires numpy >= 1.7