Every point in trj will belong to exactly one of the returned
sections. Note that this function will happily create single point
trajectories.
TrajSplit(trj, idx)A list containing one or more trajectories. The first trajectory in
  the list contains the first points from trj. Remaining trajectories
  contain the points starting from each of the idx values, in
  ascending order.
The trajectory to be split
Indices of splits. Each new section starts at one of these indices.
TrajMerge, TrajSplitAtFirstCrossing