fix non-integer division problem for Python 3
fix non-integer division problem for Python 3

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