migrate eigen-config.cmake to the new cmake_modules package

[wjwwood]

Also we should convert it to a FindEigen.cmake module.

This will involve finding and fixing anyone who uses the existing catkin provided CMake config for Eigen.

[wjwwood]

https://github.com/ros/cmake_modules

[wjwwood]

One of the issues with doing this is that people currently use it via catkin so moving it would break things. We can do this for Indigo, but for Hydro and Groovy we can only add it to cmake_modules.

[dirk-thomas]

Ticket filled for cmake_modules to readd the same functionality there: ros/cmake_modules#10

[dirk-thomas]

Closed by a94d041