Upgrade to Eigen >=3.4.1 to avoid building from source

[forderud]

The project is currently building Eigen 3.4 from source to get some SSE optimizations that were not included in the 3.4.0 release.

TASK when Eigen >=3.4.1 is released an available as an Ubuntu package:

• Replace custom Eigen build with Ubuntu libeigen3-dev package.