Switch IMU controller package to simulate real IMU

[nancyhong123]

Currently our model uses the model plugin GazeboRosImu to simulate the IMU measurements. However this is not representative of the real model as IMU should be simulated as a sensor which takes in data from the world.

The correct implementation should be to use the sensor plugin GazeboRosImuSensor

The Link below gives an example of how this can be added in a urdf:
http://gazebosim.org/tutorials?tut=ros_gzplugins