Cleanup code and use composable nodes for Gazebo

[Amronos]

I have currently made these changes only for the diff_drive_controller demo and will propagate them to the other demos if they look good.

Demos completed:

• cart_example_position
• cart_example_velocity
• cart_example_effort
• diff_drive
• tricycle_drive
• ackermann_drive
• mecanum_drive
• gripper_mimic_joint_example_position
• gripper_mimic_joint_example_effort
• pendulum_example_effort
• pendulum_example_position

[christophfroehlich]

I'm not familiar with the composable nodes of gazebo. How does #375 (comment) apply then? If we advertise this here, should we fix what is mentioned there before?

[Amronos]

I'm not familiar with the composable nodes of gazebo. How does #375 (comment) apply then? If we advertise this here, should we fix what is mentioned there before?

TBH, I am not really sure about that. From my testing passing parameters through URDF/SDF works just fine when using composable nodes (there was #441 which was fixed with #442).
I think #166 and #196 are more related to using multiple gz_ros2_control plugins and passing command line arguments.
@azeey can you provide some help here?

[christophfroehlich]

The example_mobile_robots part is fine, can you please split this up in separate PRs?

About os.path.join see the conversation there.

[Amronos]

The example_mobile_robots part is fine, can you please split this up in separate PRs?

Will do.

[Amronos]

Shifted the cmd_vel unification to #530.

[Amronos]

Shifted the addition of odometry plugin to #531.