Change CartesianPoints to be a Quantity subclass
Change CartesianPoints to be a Quantity subclass

closes #1368

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