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/QR/Tridiagonalization.h

@@ -25,7 +25,9 @@
 #ifndef EIGEN_TRIDIAGONALIZATION_H
 #define EIGEN_TRIDIAGONALIZATION_H
 
-/** \class Tridiagonalization
+/** \ingroup QR_Module
+  *
+  * \class Tridiagonalization
   *
   * \brief Trigiagonal decomposition of a selfadjoint matrix
   *