CH Robotics UM6 IMU/AHRS driver

Drivers for CH Robotic UM6 IMU/AHRS added (see www.chrobotics.com). Generic AHRS for all external AHRS systems (such as GX3 etc) added too.

The IMU driver is responsible for initializing and communicating with the IMU (e.g. getting new measurements) and updating AHRS with imu rate and estimated attitude quaternions.
AHRS subsystems in this case only updates the vehicle attitude according to given IMU rate and quaternions. 

Use as 
<subsystem name="ahrs"          type="extern_euler"/>
<subsystem name="imu"           type="um6"/>

AHRS can be renamed from "Extern_euler", I just couldnt figure out a better name.