[vtkusers] streamline & streakline

[Randy Heiland]

Streamlines are considered to be independent of time; streaklines represent
time-dependent vector fields.  Imagine a non-stationary flow, i.e. the
vector field representing the flow changes at each delta-t.  A streamline
would be computed/displayed throughout the spatial field, but just at a
snapshot in time.  A streakline, on the other hand, would begin at some
starting position (in space & time) and would vary from a streamline when
the next delta-t (and accompanying vector field) was known, then again at
the next delta-t, etc.

--Randy