Publish computed hydrodynamic forces

[acxz]

In https://github.com/osrf/vrx/blob/master/usv_gazebo_plugins/src/usv_gazebo_dynamics_plugin.cc many kinds of hydrodynamic forces are computed such as drag, added mass, buoyancy (hydrostatic), etc. It would nice if these forces could be provided to the user for debugging purposes.

UUV simulator does this via a debug flag and storing the components of the cumulative hydrodynamic forces in a vector which is eventually propagated down to a ros publisher. See here and here and Having this feature in VRX would make working with these forces easier and also provide more information to the user if they so desire.