hebiros_gazebo_plugin subscribes to Imu Plugin at fixed frequency

[josephcoombe]

See https://github.com/HebiRobotics/HEBI-ROS/blob/master/hebiros_gazebo_plugin/include/hebiros_gazebo_joint.cpp#L11-L13

this->imu_subscriber = n->subscribe<sensor_msgs::Imu>(
"hebiros_gazebo_plugin/imu/"+name,
100, boost::bind(&HebirosGazeboJoint::SubIMU, this, _1));

^ this should probably be dependent on feedback frequency.