add todo and fix default_differential attr
add todo and fix default_differential attr

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