Added triangular assignement, e.g.:

m.upper() = a+b;
only updates the upper triangular part of m.
Note that:
 m = (a+b).upper();
updates all coefficients of m (but half of the additions
will be skiped)

Updated back/forward substitution to better use Eigen's capability.

Eigen/Core

@@ -52,6 +52,7 @@ namespace Eigen {
 #include "src/Core/Swap.h"
 #include "src/Core/CommaInitializer.h"
 #include "src/Core/Triangular.h"
+#include "src/Core/TriangularAssign.h"
 
 } // namespace Eigen