Use world origin spherical coordinates from SDF if set

[chapulina]

• Part of DEM of Portuguese Ledge #146

---

Originally, WorldCommPlugin would set the world origin to coincide with the first spawned vehicle and ignore the coordinates set from SDF. This is useful for generic simulation worlds that can be used for multiple scenarios.

But for #146 , we're creating a world that has environment features that need to be precisely geolocated. This means that we need to set the world origin before any vehicles are spawned.

This PR makes sure we support both cases:

• If <sperical_coordinates> isn't set from SDF, it will be set when the first vehicle is spawned.
• If it is set, then all vehicles are spawned w.r.t. that origin

Test it

Case when there's on <spherical_coordinates>:

1. ign gazebo empty_environment.sdf
2. See on inspector that there are no spherical coordinates yet
3. bin/LRAUV
4. The vehicle is spawned at the origin and the world now has spherical coordinates

Case when there's <spherical_coordinates>:

1. Edit empty_environment.sdf to add spherical coordinates:

   diff --git a/lrauv_ignition_plugins/worlds/empty_environment.sdf b/lrauv_ignition_plugins/worlds/empty_environment.sdf
   index ce76e0b..2a2f914 100644
   --- a/lrauv_ignition_plugins/worlds/empty_environment.sdf
   +++ b/lrauv_ignition_plugins/worlds/empty_environment.sdf
   @@ -56,6 +56,16 @@       
          name="tethys::TimeAnalysisPlugin">
        </plugin-->       
                                                                  
   +    <spherical_coordinates>                                                                                                                   
   +      <surface_model>EARTH_WGS84</surface_model>                                                                                              
   +      <world_frame_orientation>ENU</world_frame_orientation>                                                                                  
   +      <latitude_deg>36.803500000000</latitude_deg>                                                                                                                                                           
   +      <longitude_deg>-121.822200000000</longitude_deg>           
   +      <elevation>0</elevation>                                                                                                                
   +      <heading_deg>0</heading_deg>
   +    </spherical_coordinates>                                       
   +                                                                   
        <plugin                                                        
          filename="ScienceSensorsSystem"                    
          name="tethys::ScienceSensorsSystem">

2. ign gazebo empty_environment.sdf
3. See on inspector that there are spherical coordinates
4. bin/LRAUV
5. The vehicle is spawned far from the origin

[hidmic]

LGTM though I haven't tested it locally just yet.

[hidmic]

I tested it locally, this is ready to go @chapulina !

[chapulina]

CI is broken for other reasons, but the spawn test passed. So merging.