Added Triangular expression to extract upper or lower (strictly or not)

part of a matrix. Triangular also provide an optimised method for forward
and backward substitution. Further optimizations regarding assignments and
products might come later.

Updated determinant() to take into account triangular matrices.

Started the QR module with a QR decompostion algorithm.
Help needed to build a QR algorithm (eigen solver) based on it.

Eigen/QR

@@ -0,0 +1,12 @@
+#ifndef EIGEN_QR_MODULE_H
+#define EIGEN_QR_MODULE_H
+
+#include "Core"
+
+namespace Eigen {
+
+#include "Eigen/src/QR/QR.h"
+
+} // namespace Eigen
+
+#endif // EIGEN_QR_MODULE_H