Add an option to make Python prints visible

[RemiFabre]

By default, a Python print will not be seen on the console when using a ros2 launch command.
Link to a discussion on the topic:
ros2/launch#188

This is problematic when debugging a lib that has no business having a ros logger added to it.
There is a simple fix. Currenlty the nodes are defined like this in the launch file:

kinematics_node = LifecycleNode(
        name="kinematics",
        namespace="",
        package="reachy_kdl_kinematics",
        executable="reachy_kdl_kinematics",
    )

But we define them like this, prints will be visible:

kinematics_node = LifecycleNode(
        name="kinematics",
        namespace="",
        package="reachy_kdl_kinematics",
        executable="reachy_kdl_kinematics",
        output="screen",
        emulate_tty=True,
    )

This should NOT be the default setting, as it adds other issues.
Options:

• There could be a flag to activate this debug mode for debug sessions for [all/one/a list of] nodes.
• Do nothing and keep this as a tip we use when in need.