Split Rotation.h to Rotation2D.h and RotationBase.h,

and more code factorization based on RotationBase.
Added notes about the main aim of the Translation and Scaling classes.

Eigen/src/Core/util/ForwardDeclarations.h

@@ -100,6 +100,7 @@ template<typename MatrixType> class Cholesky;
 template<typename MatrixType> class CholeskyWithoutSquareRoot;
 
 // Geometry module:
+template<typename Derived, int _Dim> class RotationBase;
 template<typename Lhs, typename Rhs> class Cross;
 template<typename Scalar> class Quaternion;
 template<typename Scalar> class Rotation2D;