Add flipper torque limit control

[peci1]

This PR implements the so-far-missing support for torque limit control for robots with flippers. Having this allows making much better controllers.

Depends on gazebosim/gz-sim#847, gazebosim/gz-physics#260 and maybe gazebo-forks/dart#26 .

The code explicitly checks if the torque limit isn't larger than the <joint effort=""> attribute in SDF, so it shouldn't be possible to create unrealistically strong flippers.

How to test:

LANG=C ign launch -v4 competition.ign robotName1:=X1 robotConfig1:=CTU_CRAS_NORLAB_MARV_SENSOR_CONFIG_1 worldName:=simple_cave_01 circuit:=cave

rostopic pub -1 /X1/flippers_cmd_pos/front_right std_msgs/Float64 "data: 1.0" &
rostopic pub -1 /X1/flippers_cmd_pos/front_left std_msgs/Float64 "data: 1.0" &
rostopic pub -1 /X1/flippers_cmd_pos/rear_right std_msgs/Float64 "data: -1.0" &
rostopic pub -1 /X1/flippers_cmd_pos/rear_left std_msgs/Float64 "data: -1.0" &

# the robot should stand up

sleep 10

rostopic pub -1 /X1/flippers_cmd_max_torque/front_right std_msgs/Float64 "data: 2.0" &
rostopic pub -1 /X1/flippers_cmd_max_torque/front_left std_msgs/Float64 "data: 2.0" &

# front flippers should lay down

sleep 10

rostopic pub -1 /X1/flippers_cmd_max_torque/front_right std_msgs/Float64 "data: 200.0" &
rostopic pub -1 /X1/flippers_cmd_max_torque/front_left std_msgs/Float64 "data: 200.0" &

# it should stand up again

[peci1]

flipper_torque.mp4.mp4

[peci1]

I've updated this PR with master branch. I also added validTopic() calls around the parsed custom topic names. Last, I also added a new topic for configuring the maximum velocity used for achieving positional commands. By default, the maximum joint velocity is used.

[azeey]

Works for me. Only one comment about avoiding race conditions.