* 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.

test/geometry.cpp

@@ -116,6 +116,9 @@ template<typename Scalar> void geometry(void)
   t1.translate(-v0);
 
   VERIFY((t0.matrix() * t1.matrix()).isIdentity());
+
+  t1.fromPositionOrientationScale(v0, q1, v1);
+  VERIFY_IS_APPROX(t1.matrix(), t0.matrix());
 }
 
 void test_geometry()