[RF] Improve RooCurve's chi2 test using Kahan summation.
[RF] Improve RooCurve's chi2 test using Kahan summation.

olifre/root,olifre/root,olifre/root,olifre/root,olifre/root,olifre/root,olifre/root,olifre/root,olifre/root,olifre/root,olifre/root