AC_AttitudeControl_Heli: passthrough_bf_roll_pitch_rate_yaw use quaterion to update attitude target

[IamPete1]

This is a update to remove lots of uses of _euler_angle_target this does two things:

• Switching to use quaternions means we don't need to worry about euler wrap handling
• This keeps the quaternion attitude target _attitude_target up to date rather than just the euler one _euler_angle_target.

As far as I can tell this should make no difference because neither attitude targets are actually used for anything other than logging while in roll/pitch flybar passthrough.

[MattKear]

I am fairly sure that this won't change the motor outputs, but it would be nice to confirm with the heli motors comparison tool