allow unit-less distance scale and change default
allow unit-less distance scale and change default

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