Further simplify BaseCoordinateFrame initializer
Further simplify BaseCoordinateFrame initializer

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