Rewrite FlatLambdaCDM._comoving_distance_z1z2 for Or=0 case.
Rewrite FlatLambdaCDM._comoving_distance_z1z2 for Or=0 case.

The comoving distance in a FlatLambdaCDM cosmology with just OM, OL (Or=0)
has a solution in the form of elliptic integrals.  These special functions
are available in scipy.

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