Specialize boost::python::extract template for references to Eigen matrices

* bp::extract will construct an Eigen::Ref on the fly as an rvalue using the registered converter
* move overload_base_get_item_for_std_vector from pinocchio to here -- changes behaviour of __getitem__
* redef exposeStdVectorEigenSpecificType to overload the boost vector_indexing_suite __getitem__
* move exposeStdVectorEigenSpecificType to std-vector.hpp header