hector_gazebo_plugins: added scale error to the sensor model and removed linearization in drift update

The scale error is assumed to be constant and its value is loaded from the `scaleError` plugin parameter.
The default scale error is 1.0 (no scale error).
The value returned by the model is `(real_value * scale_error) + offset + drift + noise`.
hector_gazebo_plugins: added bias publisher to gazebo_ros_imu

...to compare hector_pose_estimation estimates with ground truth.
Also renamed heading to yaw in gazebo_ros_imu and updated pseudo AHRS orientation calculation.
hector_gazebo_plugins: added initial bias