[rotorcraft] set guidance_v_zd_sp to zero in altitude hold for visualization
[math] add FLOAT_QUAT_OF_ORIENTATION_VECT
[rotorcraft][guidance][stabilization] quat setpoint fixes

Don't pretend that the commands from guidance_h are actually real euler angles.
Compose a roll/pitch quaternion from simultaneous rotation of roll/pitch,
then rotate around resulting body z-axis to align the heading.

This should "fix" the setpoint passed to the attitude stabilization if in HOVER or NAV
at large angles.
Only tested quickly in simulation...
[rotorcraft] guidance_h: normalize psi sp from nav_heading
[rotorcraft][stabilization] calculate correct yaw from guidance psi setpoint

Instead of using the psi setpoint angle to rotate around the body z-axis,
calculate the real angle needed to align the projection of the body x-axis onto the horizontal plane
with the psi setpoint.
Only makes a few degrees of difference at high roll/pitch angles, but should be correc now.
still needs to be optimized...
[rotorcraft] add FAILSAFE_ON_BAT_CRITICAL

set FAILSAFE_ON_BAT_CRITICAL to TRUE if you want to go into FAILSAFE mode (neutral attitude, slowly descend)
if battery status is critical (vsupply voltage below CRITIC_BAT_LEVEL for a while)
Merge pull request #494 'guidance_h_quat'

Don't pretend that the commands from guidance_h are actually real euler angles.
Compose a roll/pitch quaternion from simultaneous rotation of roll/pitch,
then rotate around resulting body z-axis to align the heading.

This should "fix" the setpoint passed to the attitude stabilization if in HOVER or NAV
at large angles and in cases of where heading_sp != current heading.