RF: use Cython custom routine instead of einsum
RF: use Cython custom routine instead of einsum

einsum made us require numpy 1.6.0 or greater.  The Cython routine is
faster too.

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