add MatrixBase::stableNorm() avoiding over/under-flow

using it in QR reduced the error of Keir test from 1e-12 to 1e-24 but
that's much more expensive !

Eigen/src/Core/MatrixBase.h

@@ -351,6 +351,7 @@ template<typename Derived> class MatrixBase
     Scalar dot(const MatrixBase<OtherDerived>& other) const;
     RealScalar squaredNorm() const;
     RealScalar norm()  const;
+    RealScalar stableNorm()  const;
     const PlainMatrixType normalized() const;
     void normalize();