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/streamLine/streamLineBase.C

@@ -139,17 +139,16 @@ Foam::functionObjects::streamLineBase::wallPatch() const
 }
 
 
-void Foam::functionObjects::streamLineBase::initInterpolations
+Foam::refPtr<Foam::interpolation<Foam::vector>>
+Foam::functionObjects::streamLineBase::initInterpolations
 (
     const label nSeeds,
-    label& UIndex,
-    PtrList<volScalarField>& vsFlds,
     PtrList<interpolation<scalar>>& vsInterp,
-    PtrList<volVectorField>& vvFlds,
     PtrList<interpolation<vector>>& vvInterp
 )
 {
-    // Read fields
+    refPtr<interpolation<vector>> UInterp;
+
     label nScalar = 0;
     label nVector = 0;
 
@@ -166,7 +165,7 @@ void Foam::functionObjects::streamLineBase::initInterpolations
         else
         {
             FatalErrorInFunction
-                << "Cannot find field " << fieldName << nl
+                << "Cannot find scalar/vector field " << fieldName << nl
                 << "Valid scalar fields: "
                 << flatOutput(mesh_.sortedNames<volScalarField>()) << nl
                 << "Valid vector fields: "
@@ -174,9 +173,9 @@ void Foam::functionObjects::streamLineBase::initInterpolations
                 << exit(FatalError);
         }
     }
-    vsInterp.setSize(nScalar);
+    vsInterp.resize(nScalar);
+    vvInterp.resize(nVector);
     nScalar = 0;
-    vvInterp.setSize(nVector);
     nVector = 0;
 
     for (const word& fieldName : fields_)
@@ -186,76 +185,77 @@ void Foam::functionObjects::streamLineBase::initInterpolations
             const volScalarField& f = lookupObject<volScalarField>(fieldName);
             vsInterp.set
             (
-                nScalar++,
-                interpolation<scalar>::New
-                (
-                    interpolationScheme_,
-                    f
-                )
+                nScalar,
+                interpolation<scalar>::New(interpolationScheme_, f)
             );
+            ++nScalar;
         }
         else if (foundObject<volVectorField>(fieldName))
         {
             const volVectorField& f = lookupObject<volVectorField>(fieldName);
 
-            if (f.name() == UName_)
-            {
-                UIndex = nVector;
-            }
-
             vvInterp.set
             (
-                nVector++,
-                interpolation<vector>::New
-                (
-                    interpolationScheme_,
-                    f
-                )
+                nVector,
+                interpolation<vector>::New(interpolationScheme_, f)
             );
+
+            if (f.name() == UName_)
+            {
+                // Velocity is part of sampled velocity fields
+                UInterp.cref(vvInterp[nVector]);
+            }
+
+            ++nVector;
         }
     }
 
+    if (!UInterp)
+    {
+        // Velocity was not in sampled velocity fields
+        UInterp.reset
+        (
+            interpolation<vector>::New
+            (
+                interpolationScheme_,
+                // Fatal if missing
+                lookupObject<volVectorField>(UName_)
+            )
+        );
+    }
+
     // Store the names
-    scalarNames_.setSize(vsInterp.size());
+    scalarNames_.resize(vsInterp.size());
     forAll(vsInterp, i)
     {
         scalarNames_[i] = vsInterp[i].psi().name();
     }
-    vectorNames_.setSize(vvInterp.size());
+    vectorNames_.resize(vvInterp.size());
     forAll(vvInterp, i)
     {
         vectorNames_[i] = vvInterp[i].psi().name();
     }
 
-    // Check that we know the index of U in the interpolators.
-
-    if (UIndex == -1)
-    {
-        FatalErrorInFunction
-            << "Cannot find field to move particles with : " << UName_ << nl
-            << "This field has to be present in the sampled fields " << fields_
-            << " and in the objectRegistry."
-            << exit(FatalError);
-    }
-
     // Sampled data
     // ~~~~~~~~~~~~
 
     // Size to maximum expected sizes.
     allTracks_.clear();
     allTracks_.setCapacity(nSeeds);
-    allScalars_.setSize(vsInterp.size());
+    allScalars_.resize(vsInterp.size());
     forAll(allScalars_, i)
     {
         allScalars_[i].clear();
         allScalars_[i].setCapacity(nSeeds);
     }
-    allVectors_.setSize(vvInterp.size());
+    allVectors_.resize(vvInterp.size());
     forAll(allVectors_, i)
     {
         allVectors_[i].clear();
         allVectors_[i].setCapacity(nSeeds);
     }
+
+    return UInterp;
 }
 
 
@@ -877,22 +877,13 @@ bool Foam::functionObjects::streamLineBase::read(const dictionary& dict)
     Info<< type() << " " << name() << ":" << nl;
 
     UName_ = dict.getOrDefault<word>("U", "U");
+    Info<< "    Employing velocity field " << UName_ << endl;
 
     if (fields_.empty())
     {
         dict.readEntry("fields", fields_);
-
-        if (!fields_.found(UName_))
-        {
-            FatalIOErrorInFunction(dict)
-                << "Velocity field for tracking " << UName_
-                << " should be present in the list of fields " << fields_
-                << exit(FatalIOError);
-        }
     }
 
-    Info<< "    Employing velocity field " << UName_ << endl;
-
     bool trackForward;
     if (dict.readIfPresent("trackForward", trackForward))
     {
@@ -912,7 +903,12 @@ bool Foam::functionObjects::streamLineBase::read(const dictionary& dict)
     }
     else
     {
-        trackDir_ = trackDirTypeNames.get("direction", dict);
+        trackDir_ = trackDirTypeNames.getOrDefault
+        (
+            "direction",
+            dict,
+            trackDirType::FORWARD
+        );
     }
     dict.readEntry("lifeTime", lifeTime_);
     if (lifeTime_ < 1)