Migrate from Eigen 3.2.x to 3.3.x

Change Libbarrett to work with Eigen 3.3.4.

The link below is helpful to understand the differences between the two versions. http://eigen.tuxfamily.org/index.php?title=3.3

• Documentation for 3.3.7 https://eigen.tuxfamily.org/dox/index.html
• Documentation for 3.2.10 http://eigen.tuxfamily.org/dox-3.2/