add in compatibility layer for u.mag('ST') and u.mag('AB')
add in compatibility layer for u.mag('ST') and u.mag('AB')

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