Cannot slice std::vector of Eigen types

[ManifoldFR]

As I noted in PR #412, the exposed std::vector<Eigen::Matrix> types cannot be sliced. I think this is due to the definition of __getitem__ in the visitor overload_base_get_item_for_std_vector here. It seems it doesn't handle the case where the underlying "index" PyObject is actually a Python slice (boost::python::slice) object.