ENH: stats: Add the power_divergence function that generalizes the
chi-squared test statistic, and allow it to be used in chi2_contingency.
Merge pull request #2559 from WarrenWeckesser/stats-chisquare-gtest

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