Add inf_like function to return de Sitter age.
Add inf_like function to return de Sitter age.

The new function handles the details of scalar vs. array redshift
and clarifies the code in _dS_age.

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