Added eigen_catkin_enforce macro. It can be called in a depending 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.
Merge branch 'feature/eigen_catkin_enforce' into gtsam_eigen