Simplify BaseCoordinateFrame initializer, add extra comments
Simplify BaseCoordinateFrame initializer, add extra comments

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