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'

tutorials/heatTransfer/buoyantBoussinesqPimpleFoam/BenardCells/system/streamlines

@@ -3,7 +3,7 @@
 nLines  24;
 start   (0 0.5 0);
 end     (9 0.5 0);
-fields  (U);
+fields  (p);
 
 #includeEtc "caseDicts/postProcessing/visualization/streamlines.cfg"