Make sigma_lower and sigma_upper None by default
Make sigma_lower and sigma_upper None by default

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