Backwards incompatibility: update GPS coordinates in custom simulation worlds

[klaxalk]

With the newest update of PX4 firmware and SITL for Gazebo comes a new feature:
GPS coordinates defined in the .world file for Gazebo are now actually used:

    <spherical_coordinates>
      <surface_model>EARTH_WGS84</surface_model>
      <latitude_deg>47.397743</latitude_deg>
      <longitude_deg>8.545594</longitude_deg>
      <elevation>0.0</elevation>
      <heading_deg>0</heading_deg>
    </spherical_coordinates>

Up until now, the placement of the world into the GPS frame (Zurich) has been hardcoded in the SITL and the coordinates in the .world file did not matter. But if you created a custom world prior to this update, the numbers in the .world file are different. Please update them to the values above, to match the originally used coordinates of Zurich.

You of course free to set them to any place you want. In such a case, please also mind setting corresponding local origin coordinates in the world config file for the MRS UAV system: https://github.com/ctu-mrs/mrs_uav_general/tree/master/config/worlds.