add IMU sensor

[BruceMty]

To test GPS:

To test: type these commands in each of three command windows:

• roslaunch uuv_gazebo_worlds ocean_waves.launch
• roslaunch glider_hybrid_whoi_gazebo start_demo_teleop.launch joy_id:=0
• rostopic echo glider_hybrid_whoi/fix

Then Raise the glider to an elevation >= -0.1 meters to see topic data.

Two topics are published when elevation >= -0.1: glider_hybrid_whoi/fix and glider_hybrid_whoi/fix_velocity

Elevation is hardcoded in file glider_hybrid_whoi/hybrid_glider_gazebo_plugins/hybrid_glider_gazebo_ros_plugins/src/UnderwaterGPSROSPlugin.cc. If desired, we can change this to 0.0 or make it selectable.

To test IMU:

• roslaunch uuv_gazebo_worlds ocean_waves.launch
• roslaunch glider_hybrid_whoi_gazebo start_demo_teleop.launch joy_id:=0
• rostopic echo sensors/hector_imu

IMU services are also available, type rosservice list | grep hector_imu.

Note: we still need to calibrate these sensors.

[woensug-choi]

The commit contents are copied to an issue https://github.com/Field-Robotics-Lab/glider_hybrid_whoi/issues/2