added non-negativity constraint
added non-negativity constraint

demianw/dipy,villalonreina/dipy,matthieudumont/dipy,villalonreina/dipy,JohnGriffiths/dipy,FrancoisRheaultUS/dipy,StongeEtienne/dipy,FrancoisRheaultUS/dipy,demianw/dipy,nilgoyyou/dipy,JohnGriffiths/dipy,nilgoyyou/dipy,StongeEtienne/dipy,matthieudumont/dipy