GPS bias integration mistake

[robin-shaun]

Hello, I think the expression of the GPS bias integration mistake in gazebo_gps_plugin.cpp is wrong.

  // gps bias integration
  gps_bias_.X() += random_walk_gps_.X() * dt - gps_bias_.X() / gps_corellation_time_;
  gps_bias_.Y() += random_walk_gps_.Y() * dt - gps_bias_.Y() / gps_corellation_time_;
  gps_bias_.Z() += random_walk_gps_.Z() * dt - gps_bias_.Z() / gps_corellation_time_;

It should be

  // gps bias integration
  gps_bias_.X() += random_walk_gps_.X() - gps_bias_.X() / gps_corellation_time_ * dt;
  gps_bias_.Y() += random_walk_gps_.Y() - gps_bias_.Y() / gps_corellation_time_ * dt;
  gps_bias_.Z() += random_walk_gps_.Z() - gps_bias_.Z() / gps_corellation_time_ * dt;

or the base unit doesn't match

[Jaeyoung-Lim]

@robin-shaun Thank you for your insights.

If you think this can be improved, it would be great if you can create a pull request