implement pickle in python binding

[yannrichet]

needed for python frontend like dataiku, which stores python model using pickle...
pybind11 should provide most of api :

py::class_<Archive>(m, "Archive")
    .def(py::pickle(
        [](const Archive& a) { // dump
            return py::make_tuple(a.member1, a.member2, a.member3);
        },
        [](py::tuple t) { // load
            return Archive{t[0].cast<T>(), t[1].cast<T>(), t[2].cast<T>()};
        }
    ));

[yannrichet]

more details in https://pybind11.readthedocs.io/en/stable/advanced/classes.html

py::class_<Pickleable>(m, "Pickleable")
    .def(py::init<std::string>())
    .def("value", &Pickleable::value)
    .def("extra", &Pickleable::extra)
    .def("setExtra", &Pickleable::setExtra)
    .def(py::pickle(
        [](const Pickleable &p) { // __getstate__
            /* Return a tuple that fully encodes the state of the object */
            return py::make_tuple(p.value(), p.extra());
        },
        [](py::tuple t) { // __setstate__
            if (t.size() != 2)
                throw std::runtime_error("Invalid state!");

            /* Create a new C++ instance */
            Pickleable p(t[0].cast<std::string>());

            /* Assign any additional state */
            p.setExtra(t[1].cast<int>());

            return p;
        }
    ));