Allow fitter(s) to work with data that have a length-1 model set axis.
Allow fitter(s) to work with data that have a length-1 model set axis.

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