Added new tests. Added a check to verify there is at least one streamline to do iteration in VTK file format.
Fixed loading empty TRK and TCK.
Fix to avoid crashing when loading empty TCK file.
Corrected the return type of the empty load_all() return.
Merge pull request #14 from jchoude/bf-13-load_all_empty_tck

Fix call to load_all() on empty TCK file