Apply external link forces

[Diego Ferigo]

The Physics system already requires the physics engine to support ignition::physics::AddLinkExternalForceTorque:

gym-ignition/plugins/Physics/Physics.cpp

Line 118 in 86a77cc

 ignition::physics::AddLinkExternalForceTorque, 

and used here below:

gym-ignition/plugins/Physics/Physics.cpp

Lines 685 to 698 in 86a77cc

 // Link wrenches 
 _ecm.Each<components::ExternalWorldWrenchCmd>( 
     [&](const Entity& _entity, const components::ExternalWorldWrenchCmd* _wrenchComp) { 
         auto linkIt = this->entityLinkMap.find(_entity); 
         if (linkIt == this->entityLinkMap.end()) 
             return true; 
  
         math::Vector3 force = msgs::Convert(_wrenchComp->Data().force()); 
         math::Vector3 torque = msgs::Convert(_wrenchComp->Data().torque()); 
         linkIt->second->AddExternalForce(math::eigen3::convert(force)); 
         linkIt->second->AddExternalTorque(math::eigen3::convert(torque)); 
  
         return true; 
     }); 

We should add a new ExternalForceTorque component and extend the robot_links interface.