ROS2 code generation for OCP solvers

Currently, ROS nodes support the following messages

```
float64[] parameter # parameter p (mandatory)
float64[] initial_guess # u0 (optional/recommended)
float64[] initial_y # y0 (optional)
float64 initial_penalty # initial penalty (optional)
```

which means that the called needs to specify the `parameter`, e.g.,

```bash
ros2 topic pub --once /{{ros.subscriber_subtopic}} \
{{ros.package_name}}/msg/OptimizationParameters \
"{parameter: [YOUR_PARAMETER_VECTOR], \
initial_guess: [INITIAL_GUESS_OPTIONAL], \
initial_y: [], initial_penalty: 15.0}"
```

It will be a lot more convenient if the parameters are replaced by OCP-type parameters (e.g., `x0`), when the solver is generated using the [`ocp`](https://alphaville.github.io/optimization-engine/docs/python-ocp-1) module.