RF: avoid copy if possible in Cython routine
RF: avoid copy if possible in Cython routine

From suggestion by Bago

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