Clarify impereal mass units and add lbf, kip, and psi
Clarify impereal mass units and add lbf, kip, and psi

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