[Rivet] Eigen2

[Frank Siegert]

With the recent addition of Eigen2, our pyext/ subdirectory wouldn't 
compile anymore. This was easily fixable, but there is a more serious 
problem:

Since Eigen2 isn't being installed with Rivet any plugin analysis or 
external interface using the Rivet library will break, because they 
include Analysis.hh which includes Rivet.hh which [... Matrices.hh, 
VectorN.hh ...] which includes Eigen/Core (which isn't being installed).

Is there any plan to install Eigen2 or not include it in any headers?

Frank