Add references for elliptic solution.
Add references for elliptic solution.

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