TST - Added test for cart2sphere or [0, 0, 0].
TST - Added test for cart2sphere or [0, 0, 0].

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