Log Message:
Update eigen3 to 3.3.4

Changes that might impact existing code

   Eigen 3.3 fixes a few shortcomings that might impact existing code:
     * Eigen 3.3 clarifies the usage of the Index type in SparseMatrix,
       PermutationMatrix, Transpositions, as detailed below.
     * The normalize and normalized methods will now leave their input
       unchanged if its norm is 0 or too close to 0. Previously this
       resulted in a vector containing NaNs or infinities.
     * internal::significant_decimals_impl is deprecated and users of
       custom scalar types are encouraged to overload
       NumTraits<>::digits10().

In Eigen 3.3, the evaluation mechanism of expressions has been
completely rewritten.  c.f. http://eigen.tuxfamily.org/index.php?title=3.3

Looks as though C++11 support was added and Eigen 2 support removed.

Detailed changelog:
http://eigen.tuxfamily.org/index.php?title=ChangeLog#Eigen_3.3.4