BF: fix Cython TypeError from negative indices to tuples
BF: fix Cython TypeError from negative indices to tuples

It looks as though Cython now raises a TypeError if you use negative indices
with the boundscheck decorator, even if these indices are to tuples.

Use positive indices instead.

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