Added eigen_catkin_enforce macro.

It can be called in a depender project's CMakeLists.txt file (below findpackage eigen_catkin or the catkin_simple() call) to ensure eigen_catkin's lib Eigen will be included instead of any other possible candidate (like a system installation of Eigen).
It is possible to get that right by careful treatment of workspace layer order but calling this macro is easier.

@simonlynen : The former plan of letting this done automatically by the eigen-extras.cmake.in is impossible :(, as catkin is reordering include files after all extras are run! It will enforce a so called "workspace order". And this can only be influenced by the order of workspace layers and people have troubles keeping this order clean.

I will pull this in to the gtsam_eigen branch. We won't need it in master but it would also not hurt. Maybe there are other use cases for this?