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/Cholesky

@@ -14,6 +14,17 @@
 
 namespace Eigen {
 
+/** \defgroup Cholesky_Module Cholesky module
+  * This module provides two variants of the Cholesky decomposition for selfadjoint (hermitian) matrices.
+  * Those decompositions are accessible via the following MatrixBase methods:
+  *  - MatrixBase::cholesky(),
+  *  - MatrixBase::choleskyNoSqrt()
+  *
+  * \code
+  * #include <Eigen/Cholesky>
+  * \endcode
+  */
+
 #include "src/Cholesky/Cholesky.h"
 #include "src/Cholesky/CholeskyWithoutSquareRoot.h"