add a JacobiRotation class wrapping the cosine-sine pair with

some convenient features (transpose, adjoint, product)

Eigen/src/Core/util/ForwardDeclarations.h

@@ -123,6 +123,7 @@ template<typename MatrixType> class SVD;
 template<typename MatrixType, unsigned int Options = 0> class JacobiSVD;
 template<typename MatrixType, int UpLo = LowerTriangular> class LLT;
 template<typename MatrixType> class LDLT;
+template<typename Scalar>     class JacobiRotation;
 
 // Geometry module:
 template<typename Derived, int _Dim> class RotationBase;