* split Product to a DiagonalProduct template specialization

to optimize matrix-diag and diag-matrix products without
  making Product over complicated.
* compilation fixes in Tridiagonalization and HessenbergDecomposition
  in the case of 2x2 matrices.
* added an Orientation2D small class with similar interface than Quaternion
  (used by Transform to handle 2D and 3D orientations seamlessly)
* added a couple of features in Transform.

Eigen/src/Core/Transpose.h

@@ -52,8 +52,6 @@ struct ei_traits<Transpose<MatrixType> >
           & ~( Like1DArrayBit | LowerTriangularBit | UpperTriangularBit)
           | (int(_MatrixTypeNested::Flags)&UpperTriangularBit ? LowerTriangularBit : 0)
           | (int(_MatrixTypeNested::Flags)&LowerTriangularBit ? UpperTriangularBit : 0),
-    // Note that the above test cannot be simplified using ^ because a diagonal matrix
-    // has both LowerTriangularBit and UpperTriangularBit and both must be preserved.
     CoeffReadCost = _MatrixTypeNested::CoeffReadCost
   };
 };