Add .perpendicular() function in Geometry module (adapted from Eigen1)

Documentation:
 * add an overview for each module.
 * add an example for .all() and Cwise::operator<

Eigen/LU

@@ -5,6 +5,17 @@
 
 namespace Eigen {
 
+/** \defgroup LU_Module LU module
+  * This module includes LU decomposition and related notions such as matrix inversion and determinant.
+  * This module defines the following MatrixBase methods:
+  *  - MatrixBase::inverse()
+  *  - MatrixBase::determinant()
+  *
+  * \code
+  * #include <Eigen/LU>
+  * \endcode
+  */
+
 #include "src/LU/Determinant.h"
 #include "src/LU/Inverse.h"