Added workaround/bugfix for the acceleration calculation in X-Plane-HIL-...

...link. This is clearly considered a workaround, which switches the calculation method (see code) given certain conditions. Those conditions are however set such that the switching between X-Plane accelerations and the re-calculated accelerations has minimal effect.

The code change is necessary because X-Plane (tested up to X-Plane v10.30 64bit) gives wrong accelerations when ground speed is ~zero.