RF: use quicker type detection for scalars
RF: use quicker type detection for scalars

Bago pointed out that the method I was using was rather slow
(``reduce(np.add, list_of scalars)``).  Use python sum instead, slightly
tweaked.  This is about 2x faster, and in the same ballpark as the
``np.result_type`` solution.

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