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/Geometry

@@ -5,7 +5,7 @@
 
 namespace Eigen {
 
-/** \defgroup Geometry
+/** \defgroup Geometry Geometry module
   * This module provides support for:
   *  - fixed-size homogeneous transformations
   *  - 2D and 3D rotations