Interpolating Up Direction

The original reason to start the new track segment class was, that the code had gotten so complex, that it was hard to add a little feature. I had the problem, that the slope up the hill used straight track segments which were so long, that you could see the car adjusting when you moved from one of the segments to the next. I now simply added an interpolation to the up direction and that problem has completely vanished. Instead I have gotten another one, it seems I haven't calculated the up direction at start and end correctly, the inclination now seems to jump. But already it is easier to find and fix those issues, as the code has been refactored into a number of function and that made it quite a bit simpler. So next week I will try to fix the inclination and then will start with some texturing issues.