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/src/Cholesky/Cholesky.h

@@ -25,7 +25,9 @@
 #ifndef EIGEN_CHOLESKY_H
 #define EIGEN_CHOLESKY_H
 
-/** \class Cholesky
+/** \ingroup Cholesky_Module
+  *
+  * \class Cholesky
   *
   * \brief Standard Cholesky decomposition of a matrix and associated features
   *

Eigen/src/Cholesky/CholeskyWithoutSquareRoot.h

@@ -25,7 +25,9 @@
 #ifndef EIGEN_CHOLESKY_WITHOUT_SQUARE_ROOT_H
 #define EIGEN_CHOLESKY_WITHOUT_SQUARE_ROOT_H
 
-/** \class CholeskyWithoutSquareRoot
+/** \ingroup Cholesky_Module
+  *
+  * \class CholeskyWithoutSquareRoot
   *
   * \brief Robust Cholesky decomposition of a matrix and associated features
   *