Iterate ptv_is trajectories instead of reading entire list at once.

Default for ``trajectories()`` and ``trajectories_ptvis()`` is still to read everything to memory, but now you can use ``iter_trajectories_ptvis()`` or ``trajectories(..., iter_allowed=True)`` to get an iterator. Then when you iterate over trajectories in a for loop, they are read only when needed and forgotten immediately after. See this used in ``save_particles_table()``.

Also in: finally, a test for reading ptv_is files.