Thruster plugin: add tests and velocity control

Migration.md

@@ -5,6 +5,11 @@ Deprecated code produces compile-time warnings. These warning serve as
 notification to users that their code should be upgraded. The next major
 release will remove the deprecated code.
 
+## Ignition Gazebo 5.2 to 5.3
+
+* If no `<namespace>` is given to the `Thruster` plugin, the namespace now
+  defaults to the model name, instead of an empty string.
+
 ## Ignition Gazebo 4.x to 5.x
 
 * Use `cli` component of `ignition-utils1`.

examples/worlds/auv_controls.sdf

@@ -1,4 +1,30 @@
 <?xml version="1.0" ?>
+<!--
+
+  Example showing various marine plugins working together for an
+  Autonomous Underwater Vehicle (AUV).
+
+  The following plugins are used:
+
+  * Buoyancy: applies buoyancy forces
+  * JointPositionController: controls the vertical and horizontal fins
+  * LiftDrag: generates forces on the vertical and horizontal fins
+  * Thruster: propels the vehicle
+  * Hydrodynamics: applies forces such as added mass, drag and coriolis
+
+  Example commands for the vehicle to move in a circle:
+
+  1. Move the rudder:
+
+      ign topic -t /model/tethys/joint/vertical_fins_joint/0/cmd_pos \
+         -m ignition.msgs.Double -p 'data: -0.17'
+
+  2. Propel the vehicle:
+
+      ign topic -t /model/tethys/joint/propeller_joint/cmd_thrust
+      -m ignition.msgs.Double -p 'data: -31'
+
+-->
 <sdf version="1.6">
   <world name="buoyant_tethys">
     <scene>
@@ -23,11 +49,6 @@
       filename="ignition-gazebo-scene-broadcaster-system"
       name="ignition::gazebo::systems::SceneBroadcaster">
     </plugin>
-    <plugin
-      filename="ignition-gazebo-contact-system"
-      name="ignition::gazebo::systems::Contact">
-    </plugin>
-
     <plugin
       filename="ignition-gazebo-buoyancy-system"
       name="ignition::gazebo::systems::Buoyancy">
@@ -143,4 +164,4 @@
     </include>
 
   </world>
-</sdf>
+</sdf>