Initialize parameters before ROS2 callbacks

[amalnanavati]

In general, it is best practice to initial all parameters before ROS interfaces that have callbacks. Because as soon as we initialize the ROS2 interfaces, they can theoretically be called, even if their necessary parameters are not yet initialized.

A real-world example I faced on a particularly slow computer:

1. __init__ created the joint state subscriber, but did not yet create self.__joint_names
2. The first joint state message was received.
3. The code crashed (because self.__joint_names) does not exist.

This PR should address issues like that.

[AndrejOrsula]

Thank you, @amalnanavati, for another contribution! Apologies for taking my time.