Update/remove eigen lib.

[anaegel]

Think about removing eigen.

[parnet]

Eigen is used by Promesh (GUI Application) .
From the current scope of plugins i have checked there seems no dependency on Eigen. (ConvectionDiffusion, NavierStokes, LuaShell, Electromagnetism, SmallStrainMechanics )

Promesh (GUI Application) uses feature-eigen34, the master branch of eigen seems to contain errors, maybe this should be integrated directly into Promesh or installed by ughub when the Promesh plugin installs. Promesh plugin installation should maybe removed for Examples/core since this only provides an interface for the Promesh application and to build the application QT is required which is not available on most systems