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'

src/functionObjects/field/wallBoundedStreamLine/wallBoundedStreamLine.C

@@ -6,7 +6,7 @@
      \\/     M anipulation  |
 -------------------------------------------------------------------------------
     Copyright (C) 2011-2016 OpenFOAM Foundation
-    Copyright (C) 2015-2020 OpenCFD Ltd.
+    Copyright (C) 2015-2022 OpenCFD Ltd.
 -------------------------------------------------------------------------------
 License
     This file is part of OpenFOAM.
@@ -231,23 +231,13 @@ void Foam::functionObjects::wallBoundedStreamLine::track()
     Log << type() << " : seeded " << nSeeds << " particles." << endl;
 
 
-
-    // Read or lookup fields
-    PtrList<volScalarField> vsFlds;
+    // Field interpolators
     PtrList<interpolation<scalar>> vsInterp;
-    PtrList<volVectorField> vvFlds;
     PtrList<interpolation<vector>> vvInterp;
 
-    label UIndex = -1;
-
-    initInterpolations
+    refPtr<interpolation<vector>> UInterp
     (
-        nSeeds,
-        UIndex,
-        vsFlds,
-        vsInterp,
-        vvFlds,
-        vvInterp
+        initInterpolations(nSeeds, vsInterp, vvInterp)
     );
 
     // Additional particle info
@@ -256,7 +246,7 @@ void Foam::functionObjects::wallBoundedStreamLine::track()
         particles,
         vsInterp,
         vvInterp,
-        UIndex,         // index of U in vvInterp
+        UInterp.cref(), // velocity interpolator (possibly within vvInterp)
         trackLength_,   // fixed track length
         isWallPatch,    // which faces are to follow