hector gazebo plugin for imu sensor is added

Hector Gazebo Plugin for IMU sensor is added. It publishes correct information on /imu/data topic.
There should be a corresponding `package.xml` change to `jackal_gazebo` for this dependency, I assume?

Please also file a bug to raise awareness of this issue in the gazebo upstream: https://github.com/ros-simulation/gazebo_ros_pkgs/issues