Implement RHF stability analysis as a test case.

[amjames]

The functions for doing Hessian-Vector products are exported by the SCF::HF class which is a wave-function derivative I have shown the exports below.

        .def("onel_Hx", &scf::HF::onel_Hx, "One-electron Hessian-vector products.")
        .def("twoel_Hx", &scf::HF::twoel_Hx, "Two-electron Hessian-vector products")
        .def("cphf_Hx", &scf::HF::cphf_Hx, "CPHF Hessian-vector prodcuts (4 * J - K - K.T).")

Note: This will have to wait untill after the porting to psi matrix objects ( #5 ) is finished since the interface uses Matrix objects.