Add support for Gazebo9

This fixes the problem related to Gazebo's position control, which can be seen
when dropping a position-controlled robot. The issue is fixed by using the
_preserveWorldVelocity argument introduced in Gazebo 9.0.0.

https://bitbucket.org/osrf/gazebo/pull-requests/2814/fix-issue-2111-by-providing-options-to/diff

Related issue/commit in gazebo_ros_pkgs, which introduces a similar fix:

ros-simulation/gazebo_ros_pkgs@3164e4c
ros-simulation/gazebo_ros_pkgs#479

steer_bot_hardware_gazebo/src/steer_bot_hardware_gazebo.cpp

@@ -212,7 +212,11 @@ namespace steer_bot_hardware_gazebo
         {
           pos_cmd = front_steer_jnt_pos_cmd_;
         }
+#if GAZEBO_MAJOR_VERSION >= 9
+        sim_joints_[i]->SetPosition(0, pos_cmd, true);
+#else
         sim_joints_[i]->SetPosition(0, pos_cmd);
+#endif
       }
       else if(gazebo_jnt_name == virtual_front_steer_jnt_names_[INDEX_LEFT])
       {
@@ -228,7 +232,11 @@ namespace steer_bot_hardware_gazebo
         {
           pos_cmd = 2*front_steer_jnt_pos_cmd_;
         }
+#if GAZEBO_MAJOR_VERSION >= 9
+        sim_joints_[i]->SetPosition(0, pos_cmd, true);
+#else
         sim_joints_[i]->SetPosition(0, pos_cmd);
+#endif
       }
       else
       {
@@ -461,8 +469,13 @@ namespace steer_bot_hardware_gazebo
   void SteerBotHardwareGazebo::GetCurrentState(std::vector<double>& _jnt_pos, std::vector<double>& _jnt_vel, std::vector<double>& _jnt_eff,
                                                const int _if_index, const int _sim_jnt_index)
   {
+#if GAZEBO_MAJOR_VERSION >= 9
+    _jnt_pos[_if_index] +=
+        angles::shortest_angular_distance(_jnt_pos[_if_index], sim_joints_[_sim_jnt_index]->Position(0u));
+#else
     _jnt_pos[_if_index] +=
         angles::shortest_angular_distance(_jnt_pos[_if_index], sim_joints_[_sim_jnt_index]->GetAngle(0u).Radian());
+#endif
     _jnt_vel[_if_index] = sim_joints_[_sim_jnt_index]->GetVelocity(0u);
     _jnt_eff[_if_index] = sim_joints_[_sim_jnt_index]->GetForce(0u);
   }