FindEigen3.cmake: Add reading hints of Eigen directory location from environment variables EIGEN3_ROOT and EIGEN3_ROOT_DIR .
FindEigen3.cmake: Add reading hints of Eigen directory location from environment variables EIGEN3_ROOT and EIGEN3_ROOT_DIR .

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branch : traversaro/findeigen3cmake-add-reading-hints-of-eig-1407426517521

mjbshaw/Eigen,rotorliu/eigen,mjbshaw/Eigen,rotorliu/eigen,madlib/eigen_backup,madlib/eigen_backup,mjbshaw/Eigen,madlib/eigen_backup,mjbshaw/Eigen,madlib/eigen_backup,rotorliu/eigen,rotorliu/eigen