Guidance, AHRS and INS updates

Guidance (horizontal):
- Changed the IGAIN precision. Previously the values of the IGAIN should have been in a very high range. I'm not sure if this is the correct range now, so maybe someone can look into that a little bit more?
- Introduced a VGAIN (feedforward speed), to compensate drag.

AHRS:
- Made it possible to define the course deviation. This is needed for the indoor GPS, because with fast turns the course can deviate more than the maximum.
This isn't the nicest way to implement the heading of the tracking system, but it works as supposed with minimum changes.

INS:
- Added the possibility to use the GPS altitude in the kalman filter. This is also needed for the indoor GPS, but could also be used outdoor if you trust the GPS altitude.

Next tot that I added all these functions to the ardrone2 (indoor) airframe and tuned some of these gains.