Add Eigen3

[KorigamiK]

Eigen3 is an awesome C++ template library for linear algebra: matrices, vectors, numerical solvers, and related algorithms.

It is fully inline so there should not be any static or shared libraries required to link.

I would love to know how you would go about linking this one in a cmakelists.txt :D

[KorigamiK]

I've created a minimal working example to test that it works :)

[sharkwouter]

Thanks! I'll give this a test and do a review.

[KorigamiK]

Thanks for the changes, I can't test them at the moment but please change them as you desire.

[sharkwouter]

Done, should work now. I build your example and it seemed to work.