SDF tag <elevation> should be used in world files, not <altitude>

[slgrobotics]

When running any GZ simulation, the following error pops up:

Warning [Utils.cc:130] [/sdf/world[@name="default"]/spherical_coordinates/altitude:
.../PX4-Autopilot/Tools/simulation/gz/worlds/default.sdf:L153]: 
XML Element[altitude], child of element[spherical_coordinates], not defined in SDF. 
Copying[altitude] as children of [spherical_coordinates].

This is likely due to <altitude> tag, which must be <elevation> according to SDF format specs:

http://sdformat.org/spec?ver=1.11&elem=world

When I replace the tag, error message disappears:

    <spherical_coordinates>
      <surface_model>EARTH_WGS84</surface_model>
      <world_frame_orientation>ENU</world_frame_orientation>
      <latitude_deg>33.00000</latitude_deg>
      <longitude_deg>-86.00000</longitude_deg>
      <elevation>140</elevation>
    </spherical_coordinates>
  </world>
</sdf>

It makes sense, as "altitude" usually refers to a vehicle, and "elevation" - to terrain.

@frede791 - please take a look.

[slgrobotics]

@frede791 - I cannot close it, please do. It has been merged/completed.