XSens starting before frame 0

In manual correction we sometimes want to let XSens start before frame 0. This is perfectly fine with PeTrack.

If we want to apply that transformation to the C3D file, we have a problem. We apply timeshifts via the first_frame and last_frame attributes. Hoewever, first_frame cannot be negative. Therefore, we need to use a trick.

We add dummy data at the beginning of the file and set the first_frame to 0. In reality, the real data starts with the first real frame. first_frame should therefore be interpreted as minus the number of dummy frames. It might be helpful to look at such transformed data in PeTrack as well, so we should support this "specification".