- make MatrixBase and all expressions aware of their preferred traversal order.

Honor this preference in operator=.
- add several methods to the API
- rework API for diagonal matrices
- add benchmarking code

doc/snippets/MatrixBase_block.cpp

@@ -1,4 +1,4 @@
-Matrix4d m = Matrix4d::diagonal(Vector4d(1,2,3,4));
+Matrix4d m = Vector4d(1,2,3,4).asDiagonal();
 cout << "Here is the matrix m:" << endl << m << endl;
 cout << "Here is m.block<2, 2>(2, 2):" << endl << m.block<2, 2>(2, 2) << endl;
 m.block<2, 2>(2, 0) = m.block<2, 2>(2, 2);