Update submodule sitl_gazebo to latest Thu May 28 00:38:08 UTC 2020

    - sitl_gazebo in PX4/Firmware (05856c1): PX4/PX4-SITL_gazebo-classic@2736a91
    - sitl_gazebo current upstream: PX4/PX4-SITL_gazebo-classic@ab527bf
    - Changes: PX4/PX4-SITL_gazebo-classic@2736a91...ab527bf

    ab527bf 2020-05-22 Pandacoolcool - Update gazebo_usv_dynamics_plugin.cpp (#492)
4acd679 2020-05-18 Julian Oes - Fix for Gazebo 11
f021cdd 2020-05-17 Grzegorz Araminowicz - make position_gztopic work without specifying joint_name (as it is not used anyway) (#459)
12da9e6 2020-04-22 tnkkk - Moved add_magnetic_field_covariance from OnUpdate to Load
0b348fd 2020-05-11 JaeyoungLim - Handle continuous zoom (#488)
dab2eba 2020-05-03 kamilritz - Fix wind_vel_transformation
7f1b8e2 2020-05-08 JaeyoungLim - Fix gazebo motor model plugin calculations (#484)
4c1c512 2020-05-07 JaeyoungLim - Fix tiltrotor motor tilt oscillations (#471)