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
Fix uninitialized variable warning in SparseQR

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branch : vladimir_ch/fix-uninitialized-variable-warning-in-sp-1408513228472
Merged in traversaro/eigen/traversaro/findeigen3cmake-add-reading-hints-of-eig-1407426517521 (pull request #76)

FindEigen3.cmake: Add reading hints of Eigen directory location from environment variables EIGEN3_ROOT and EIGEN3_ROOT_DIR .
Merged in vladimir_ch/eigen-1/vladimir_ch/fix-uninitialized-variable-warning-in-sp-1408513228472 (pull request #77)

Fix uninitialized variable warning in SparseQR
EIGEN_EXCEPTIONS was not defined in test/main.h, therefore all VERIFY_RAISES_ASSERT tests were not enabled