-Add sick robot day target model
-Add target model to sick robot day 2014 world
Create README.md
-Add inner block for sick robot day 2014
-Add inner block to world
-Add complete sick robot day 2014 target stations
-Update world file
-Add two target panel examples to world
-Add complete sick robot day 2014 outer ring with stations
-Add inner block with stations model
- Update world
- Adjusted lighting
hector_gazebo_plugins: fixed wrong calculation of reference earth magnetic field vector if declination!=0
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
hector_pose_estimation: calculate angular rate from quaternion difference directly

This seems to be numerically more stable and removes the jitter in the angular rate signal.
hector_gazebo_plugins: fixed invocation of sensor model in gazebo_ros_imu and gazebo_ros_sonar to also respect the scale error
hector_gazebo_plugins: added dynamic_reconfigure server for IMU sensor model parameters
hector_gazebo_plugins: added dynamic_reconfigure servers to gps, magneto and sonar plugins

The GPS plugin allows to configure the status and server flags in the output message,
additionally to the error characteristics. This allows to simulate GPS dropouts.