Port USV plugins from gazebo

[iche033]

There are a set of gazebo plugins for USVs in the VRX repo:
https://github.com/osrf/vrx/tree/master/usv_gazebo_plugins

This ticket tracks the progress of porting of those plugins to Ignition gazebo systems:

• usv_gazebo_dynamics_plugin
• usv_gazebo_thrust_plugin
• usv_gazebo_wind_plugin
• buoyancy_gazebo_plugin

Once the plugins are ported, an example world should be provided to demonstrate the support of USVs in Ignition gazebo.

Desired behavior

Ability to simulation USV models with configurable dynamics / thruster params.

Additional context

Related work is porting of the wave model to Ignition so that the USV vehicle motion is influenced by wave motion.

[chapulina]

usv_gazebo_dynamics_plugin

This looks like ignition::gazebo::systems::Hydrodynamics. It even links to the original plugin.

One thing to note is that the plugin has been added without tests in #749, I've been meaning to add some tests soon.

We also have a fork of it on LRAUV because some bugs have been fixed. Those should be upstreamed soon, progress is tracked here: osrf/lrauv#37

usv_gazebo_thrust_plugin

Maybe ignition::gazebo::systems::Thruster can be extended to support all features on that plugin.

usv_gazebo_wind_plugin

It looks similar to ignition::gazebo::systems::WindWffects, I think the functionality could be combined.

buoyancy_gazebo_plugin

I think the core functionality should be achieved with ignition::gazebo::systems::Buoyancy, but note that it's a world plugin instead of a model plugin. It also seems to have some different parameters, not sure if the drag is best kept at LiftDrag or Hydrodynamics.

[iche033]

after some discussion with @caguero. The plan is:

• try out the Hydrodynamics and Buoyancy plugin as is
• look more into the Thruster plugin and see if it supports the actuator capability we need
• port the wave model part of the usv_gazebo_dynamics_plugin to Ignition
• convert the WAM-V urdf file from VRX to sdf with a single fixed configuration

[caguero]

We'll extend the Thruster plugin to support the articulation of the thruster.