ENH: streamline improvements

- barycentric coordinates in interpolation (instead of x/y/z)

- ease U (velocity) requirement.
  Needn't be named in the sampled fields.

- default tracking direction is 'forward'

etc/caseDicts/postProcessing/visualization/streamlines.cfg

@@ -13,7 +13,7 @@ executeControl  writeTime;
 writeControl    writeTime;
 
 setFormat       vtk;
-trackForward    true;
+direction       forward;   // (forward | backward | bidirectional)
 
 lifeTime        10000;
 nSubCycle       5;