message: Renamed landmark_definitions to vertex_definitions in Python bindings
  Also updated the documentation to mention the vertex definitions.
diff: @@ -168,8 +168,8 @@ PYBIND11_MODULE(eos, eos_module)
  .value("PcaModel", morphablemodel::MorphableModel::ExpressionModelType::PcaModel);
  morphable_model
  - .def(py::init<morphablemodel::PcaModel, morphablemodel::PcaModel, cpp17::optional<std::unordered_map<std::string, int>>, std::vector<std::array<double, 2>>>(), "Create a Morphable Model from a shape and a colour PCA model, and optional texture coordinates.", py::arg("shape_model"), py::arg("color_model"), py::arg("landmark_definitions") = cpp17::nullopt, py::arg("texture_coordinates") = std::vector<std::array<double, 2>>())
  - .def(py::init<morphablemodel::PcaModel, morphablemodel::ExpressionModel, morphablemodel::PcaModel, cpp17::optional<std::unordered_map<std::string, int>>, std::vector<std::array<double, 2>>>(), "Create a Morphable Model from a shape and a colour PCA model, an expression PCA model or blendshapes, and optional texture coordinates.", py::arg("shape_model"), py::arg("expression_model"), py::arg("color_model"), py::arg("landmark_definitions") = cpp17::nullopt, py::arg("texture_coordinates") = std::vector<std::array<double, 2>>())
  + .def(py::init<morphablemodel::PcaModel, morphablemodel::PcaModel, cpp17::optional<std::unordered_map<std::string, int>>, std::vector<std::array<double, 2>>>(), "Create a Morphable Model from a shape and a colour PCA model, and optional vertex definitions and texture coordinates.", py::arg("shape_model"), py::arg("color_model"), py::arg("vertex_definitions") = cpp17::nullopt, py::arg("texture_coordinates") = std::vector<std::array<double, 2>>())
  + .def(py::init<morphablemodel::PcaModel, morphablemodel::ExpressionModel, morphablemodel::PcaModel, cpp17::optional<std::unordered_map<std::string, int>>, std::vector<std::array<double, 2>>>(), "Create a Morphable Model from a shape and a colour PCA model, an expression PCA model or blendshapes, and optional vertex definitions and texture coordinates.", py::arg("shape_model"), py::arg("expression_model"), py::arg("color_model"), py::arg("vertex_definitions") = cpp17::nullopt, py::arg("texture_coordinates") = std::vector<std::array<double, 2>>())
  .def("get_shape_model", &morphablemodel::MorphableModel::get_shape_model, "Returns the PCA shape model of this Morphable Model.") // Not sure if that'll really be const in Python? I think Python does a copy each time this gets called?
  .def("get_color_model", &morphablemodel::MorphableModel::get_color_model, "Returns the PCA colour (albedo) model of this Morphable Model.")
  .def("get_expression_model", &morphablemodel::MorphableModel::get_expression_model, "Returns the shape expression model or an empty optional if the Morphable Model does not have a separate expression model.")
  