Resolve "Head-Rotation Trajectory File: Remove Subframe-Index" [Ref-Using-Main]

Related to #173 (closed) and #174 (closed)

This MR removes the frame index from the metadata for ISM and the subframe index from the headtracking data for the rendering. This seems fine to me, but please speak up if anyone sees an issue with removing this. @vaclav and @multrus, please feel free to review.

The change is bit exact, provided of course that the ISM metadata and headtracking trajectories are updated accordingly. This MR updates the metadata and trajectories used in the self_test, and I hope I found all other examples too.