DOC: added new tutorial about volume vizualization
DOC: added new tutorial about volume vizualization

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