Improve interpolate.interp1d documentation explaining what slinear, quadratic and cubic keywords mean
MAINT: stats: removed unused line of code.
MAINT: stats: remove unused imports
Merge pull request #2630 from WarrenWeckesser/maint-stats-cleanup

MAINT: stats: a bit of clean up
Merge pull request #2628 from nils-werner/interp-docs

Explaining interpolate.interp1d interpolation method keywords
ENH: stats: Add the power_divergence function that generalizes the
chi-squared test statistic, and allow it to be used in chi2_contingency.