ptv_is reading by an iterator, so we don't have to hold everything in memory if not completely necessary.
Use the new iter_trajectories_ptvis when possible to save memory, e.g. in saving a trajectory table without loading all trajectories first.
Fix: data directory relative to project root.
Different minimal trajectory length for format that require forward difference velocity calculation (ptvis)
added the test that fails with the min length less than 2
fixed the problem with the xuap that requires min. length of 2 (not 1)
Merge pull request #4 from yosefm/iter_trajects

Iterate ptv_is trajectories instead of reading entire list at once.