Allow hydrodynamics plugin to apply to named links

[srmainwaring]

The hydrodynamics plugin currently applies buoyancy and drag forces to all links in a model. The proposal is to allow the user to specify which links should be included / excluded from the calculation. For example the contribution from sensor links is likely to be small and these could be safely excluded without a material impact to the physical behaviour.

See the gz-sim-buoyancy-system plugin and the graded_buoyancy.sdf for an example where certain models and links have the calculation enabled.

<plugin
  filename="gz-marine1-hydrodynamics-system"
  name="gz::sim::systems::Hydrodynamics">

   <!-- Hydrodynamics -->
   <hydrodynamics>
    <damping_on>1</damping_on>
    <viscous_drag_on>1</viscous_drag_on>
    <pressure_drag_on>1</pressure_drag_on>

    <!-- Linear and Angular Damping -->  
    <cDampL1>1.0E-6</cDampL1>
    <cDampL2>1.0E-6</cDampL2>
    <cDampR1>1.0E-6</cDampR1>
    <cDampR2>1.0E-6</cDampR2>

    <!-- 'Pressure' Drag -->
    <cPDrag1>1.0E+2</cPDrag1>
    <cPDrag2>1.0E+2</cPDrag2>
    <fPDrag>0.4</fPDrag>
    <cSDrag1>1.0E+2</cSDrag1>
    <cSDrag2>1.0E+2</cSDrag2>
    <fSDrag>0.4</fSDrag>
    <vRDrag>1.0</vRDrag>

    <!-- Either: enable specific links -->
    <enable>base_link</enable>
    <enable>left_hull</enable>
    <enable>right_hull</enable>

    <!-- Or: disable specific links -->
    <disable>imu_sensor_link</disable>

  </hydrodynamics>
</plugin>