Slight performance enhancement for Const models.
Slight performance enhancement for Const models.

See https://github.com/astropy/astropy/pull/3027#issuecomment-59576917

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