DOC: added simple mosaic example in tutorial viz_slice.py
DOC: added simple mosaic example in tutorial viz_slice.py

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