Set path to Eigen3 directory with -DEIGEN3_ROOT=

Let the user specify the path to the Eigen3 checkout/installation (since it is a header library, either will suffice). Also requires version 3.1 of the library.

After this changeset is applied, you should be able to get Eigen3 by doing:

    hg clone -b 3.1 https://bitbucket.org/eigen/eigen /path/to/where/you/want/eigen
    cmake .. -DEIGEN3_ROOT=/path/to/where/you/want/eigen