Separate frame reading from ptv_is/xuap trajectory construction, in preparation for reading a concatenated file.
Read single-file ptv_is data.
Sequence class handles trajectories out of frame bounds.
Add a new, much faster save format (HDF5-PyTables based) and speed up also the old npz format by checking frame bounds.
Complete documentation of new functions.
Faster sequence processing
Handle frames without tracers mildly, with a warning. The stricter policy of raising an exception is more correct, but less usable right now, until Sequence can skip frames and mark the skipped places.
Allow RBF interpolation for 1-D values.
Fix case of only one tracer in frame
Merge pull request #1 from yosefm/master

Omnibus update, nobody reviewed, don't come crying later.