Refactored bbox computing, add python3 compat.
Refactored bbox computing, add python3 compat.

rfdougherty/dipy,matthieudumont/dipy,demianw/dipy,JohnGriffiths/dipy,FrancoisRheaultUS/dipy,Messaoud-Boudjada/dipy,beni55/dipy,villalonreina/dipy,mdesco/dipy,JohnGriffiths/dipy,FrancoisRheaultUS/dipy,oesteban/dipy,Messaoud-Boudjada/dipy,jyeatman/dipy,oesteban/dipy,matthieudumont/dipy,sinkpoint/dipy,StongeEtienne/dipy,samuelstjean/dipy,demianw/dipy,StongeEtienne/dipy,mdesco/dipy,beni55/dipy,jyeatman/dipy,sinkpoint/dipy,nilgoyyou/dipy,samuelstjean/dipy,nilgoyyou/dipy,villalonreina/dipy,samuelstjean/dipy,rfdougherty/dipy