Add test for subclass that only specifies w(z) and not efunc, etc.
Add test for subclass that only specifies w(z) and not efunc, etc.

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