add example of a user-defined statistic function
add example of a user-defined statistic function

MSeifert04/astropy,mhvk/astropy,astropy/astropy,funbaker/astropy,tbabej/astropy,tbabej/astropy,mhvk/astropy,kelle/astropy,kelle/astropy,lpsinger/astropy,pllim/astropy,stargaser/astropy,saimn/astropy,larrybradley/astropy,aleksandr-bakanov/astropy,bsipocz/astropy,kelle/astropy,kelle/astropy,funbaker/astropy,dhomeier/astropy,StuartLittlefair/astropy,larrybradley/astropy,AustereCuriosity/astropy,kelle/astropy,astropy/astropy,saimn/astropy,AustereCuriosity/astropy,saimn/astropy,tbabej/astropy,mhvk/astropy,larrybradley/astropy,AustereCuriosity/astropy,AustereCuriosity/astropy,aleksandr-bakanov/astropy,DougBurke/astropy,MSeifert04/astropy,joergdietrich/astropy,dhomeier/astropy,bsipocz/astropy,joergdietrich/astropy,joergdietrich/astropy,tbabej/astropy,joergdietrich/astropy,funbaker/astropy,pllim/astropy,DougBurke/astropy,AustereCuriosity/astropy,dhomeier/astropy,larrybradley/astropy,aleksandr-bakanov/astropy,mhvk/astropy,funbaker/astropy,astropy/astropy,saimn/astropy,StuartLittlefair/astropy,bsipocz/astropy,bsipocz/astropy,astropy/astropy,astropy/astropy,aleksandr-bakanov/astropy,StuartLittlefair/astropy,saimn/astropy,DougBurke/astropy,larrybradley/astropy,lpsinger/astropy,MSeifert04/astropy,DougBurke/astropy,MSeifert04/astropy,pllim/astropy,tbabej/astropy,lpsinger/astropy,StuartLittlefair/astropy,stargaser/astropy,dhomeier/astropy,stargaser/astropy,joergdietrich/astropy,pllim/astropy,StuartLittlefair/astropy,mhvk/astropy,lpsinger/astropy,dhomeier/astropy,pllim/astropy,lpsinger/astropy,stargaser/astropy