header only integration of Eigen types

If you google around a bit, you'll see a number of users trying to integrate Eigen types with Boost.Python. It's a bit hard and the solutions are lack-luster and cannot handle all issues. Let's strive for something built-in and header based so we don't complicate other uses of Boost.Python.

Here's an incomplete list:
- http://fhtagn.net/prog/2015/04/16/quaternion_boost_python.html
- https://github.com/julienr/boost_numpy_eigen
- http://eigen.tuxfamily.org/index.php?title=PythonInteropBoost
- https://github.com/eudoxos/minieigen
- https://github.com/ndarray/ndarray (note: depends on Boost.Numpy)