Implemented a slight code simplification suggested by @mhvk (to eliminate the explicit for loop at the beginning of _prepare_input_units, and eliminate the awkward (and broken) toggling of an internal instance attribute depending on whether or not some inputs had units when the model is called.
Implemented a slight code simplification suggested by @mhvk (to eliminate the explicit for loop at the beginning of _prepare_input_units, and eliminate the awkward (and broken) toggling of an internal instance attribute depending on whether or not some inputs had units when the model is called.

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