Bugfix GPS plugin coordinate

[achille-martin]

According to uuvsimulator#390, Gazebo is publishing -East -North Up instead of East North Up.

UUV Simulator handled this case by adding a minus sign to lat / lon in the GPS ROS Plugin.

However, this approach makes the creation of a .world confusing, as the origin of the world must have negative lat / lon.

The temporary solution (until Gazebo fixes its frame output according to gazebosim/gazebo-classic#2022 (comment)) is:

<!-- In the .world file -->
<spherical_coordinates>
      <!-- currently gazebo has a bug: instead of outputing lat, long, altitude in ENU
      (x = East, y = North and z = Up) as the default configurations, it's outputting (-E)(-N)U,
      therefore we rotate the default frame 180 so that it would go back to ENU -->
      <heading_deg>180</heading_deg>
  </spherical_coordinates>

[achille-martin]

For information, this MR coincides with OCSv3.12.0 (it introduces breaking changes for old versions)