add astronomical_constants class
add astronomical_constants class

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