Depending on FindEigen3.cmake is not portable

[de-vri-es]

It was decided to deprecate the FindEigen.cmake provided in this package in favor of the "upstream" FindEigen3.cmake. However, there is no real upstream FindEigen3.cmake. It was added tot he Eigen source tree as an example, or something developers can copy for their own project. It was never meant to be installed system wide [1].

For some reason the Ubuntu packagers are installing the file in a system wide path. However, depending on that is far from portable. This guideline of depending on FindEigen3.cmake breaks at least on Arch and Gentoo, and undoubtedly more distributions.

[1] http://eigen.tuxfamily.org/bz/show_bug.cgi?id=1219

[de-vri-es]

As commented also in #25, Arch Linux backported Eigen3Config.cmake into their package. That means for me personally the problem is solved, although the current situation is still not portable: there is still no out-of-the-box find_package(Eigen3) for plain upstream Eigen 3.2.