BUG/REF: bug in ipwra method, use HC0 to match GMM and Stata
BUG/REF: bug in ipwra method, use HC0 to match GMM and Stata

bashtage/statsmodels,josef-pkt/statsmodels,josef-pkt/statsmodels,statsmodels/statsmodels,statsmodels/statsmodels,statsmodels/statsmodels,josef-pkt/statsmodels,bashtage/statsmodels,bashtage/statsmodels,josef-pkt/statsmodels,bashtage/statsmodels,josef-pkt/statsmodels,statsmodels/statsmodels,josef-pkt/statsmodels,bashtage/statsmodels,statsmodels/statsmodels,bashtage/statsmodels,statsmodels/statsmodels