Replace Eigen by classes Spinor, SpinMatrix of our own
Motivation: Eigen is so heavily templated that it is in the way of Python bindings, regardless whether we use Swig or pybind11. Eigen is meant for matrices of arbitrary size. For our SO(3) and SU(2) matrices, a few 100 lines of dedicated code are more appropriate.
Edited by Wuttke, Joachim