Make QR the default in GaussNewtonOptimizer

Theoretically QR offers the best blend of speed and numerical stability.
Empirically the QR implementation is slightly faster than the Cholesky
implementation.

git-svn-id: https://svn.apache.org/repos/asf/commons/proper/math/trunk@1569907 13f79535-47bb-0310-9956-ffa450edef68