fixed bug on fitting for data with dy=0
fixed bug on fitting for data with dy=0

lewisodriscoll/sasview,lewisodriscoll/sasview,SasView/sasview,lewisodriscoll/sasview,SasView/sasview,SasView/sasview,SasView/sasview,SasView/sasview,lewisodriscoll/sasview,lewisodriscoll/sasview,SasView/sasview