works on ARMAR-6

fixed freezing when looking down, also able to look left and right (+down) from center down position
- needs tetsing on real robot, only tested in simulation

Tamim requested the LookUp/Down etc. skills to not "weirdly" look at a combined direction (like left-down, when calling left and then down)
So the directions do not stack anymore

With Fabian Reister

fix: Transfer gaze targets to a dedicated gaze root frame (relative to head base frame); Normalize calculation of pitch depending on selected yaw.

The issue here was that the calibration using the quadratic regression worked okay for yaw=0. However, as soon as there is a small tilt of the head base frame (i.e., the robot root frame and the head root frame where not parallel as in ARMAR-6), this calculation failed and resulted in a sinuidal movement of the head when the platform was rotated.
To fix this, the gaze targets are now transformed to a gaze root frame which in turn is relative to the head frame. Additionally, the target yaw is compensated before calculating the pitch.