NullMixer - Send NAN instead of 0. NAN is mapped to disarmed in fmu and io. A Null mixer can now be used to set a fixed value: "disarmed" if the system is operational or "failsafe" in failsafe mode

FailureDetector - Add check for non-zero roll/pitch failure parameters. Rename 'result' into 'updated'

FailureDetector - New class, first working simple implementation. Outputs PX4_ERR messages if roll or pitch is exceeded.