ENH: handling of open edges distanceSurface at 0 (issue #948)

Some special adjustments are undertaken for distance = 0.

- With the isoSurfaceCell algorithm is used, additional checks for open
  surfaces edges are used to limit the extend of resulting distance
  surface. The resulting surface elements will not, however, contain
  partial cell coverage.

- Always treated as signed (ignoring the input value), since it is
  nearly impossible to generate any surface otherwise.

src/sampling/surface/isoSurface/isoSurfaceCell.C

@@ -85,6 +85,11 @@ Foam::isoSurfaceCell::cellCutType Foam::isoSurfaceCell::calcCutType
     const label celli
 ) const
 {
+    if (ignoreCells_.test(celli))
+    {
+        return NOTCUT;
+    }
+
     const cell& cFaces = mesh_.cells()[celli];
 
     if (isTet.test(celli))
@@ -174,8 +179,9 @@ Foam::isoSurfaceCell::cellCutType Foam::isoSurfaceCell::calcCutType
         {
             return SPHERE;
         }
-        else
+        else if (nCuts > 1)
         {
+            // Need at least two edge cuts, otherwise this is a spurious cut
             return CUT;
         }
     }
@@ -1268,20 +1274,22 @@ Foam::triSurface Foam::isoSurfaceCell::subsetMesh
 Foam::isoSurfaceCell::isoSurfaceCell
 (
     const polyMesh& mesh,
-    const scalarField& cVals,
-    const scalarField& pVals,
+    const scalarField& cellValues,
+    const scalarField& pointValues,
     const scalar iso,
     const bool regularise,
     const boundBox& bounds,
-    const scalar mergeTol
+    const scalar mergeTol,
+    const bitSet& ignoreCells
 )
 :
     MeshStorage(),
     mesh_(mesh),
-    cVals_(cVals),
-    pVals_(pVals),
+    cVals_(cellValues),
+    pVals_(pointValues),
     iso_(iso),
     bounds_(bounds),
+    ignoreCells_(ignoreCells),
     mergeDistance_(mergeTol*mesh.bounds().mag())
 {
     if (debug)
@@ -1298,6 +1306,8 @@ Foam::isoSurfaceCell::isoSurfaceCell
             << "    mergeTol      : " << mergeTol << nl
             << "    mesh span     : " << mesh.bounds().mag() << nl
             << "    mergeDistance : " << mergeDistance_ << nl
+            << "    ignoreCells   : " << ignoreCells_.count()
+            << " / " << cVals_.size() << nl
             << endl;
     }
 
@@ -1317,7 +1327,7 @@ Foam::isoSurfaceCell::isoSurfaceCell
 
 
     // Determine if any cut through cell
-    calcCutTypes(isTet, cVals, pVals);
+    calcCutTypes(isTet, cellValues, pointValues);
 
     if (debug && isA<fvMesh>(mesh))
     {
@@ -1361,8 +1371,8 @@ Foam::isoSurfaceCell::isoSurfaceCell
         calcSnappedCc
         (
             isTet,
-            cVals,
-            pVals,
+            cellValues,
+            pointValues,
             snappedPoints,
             snappedCc
         );
@@ -1389,8 +1399,8 @@ Foam::isoSurfaceCell::isoSurfaceCell
         calcSnappedPoint
         (
             isTet,
-            cVals,
-            pVals,
+            cellValues,
+            pointValues,
             snappedPoints,
             snappedPoint
         );
@@ -1417,8 +1427,8 @@ Foam::isoSurfaceCell::isoSurfaceCell
 
         generateTriPoints
         (
-            cVals,
-            pVals,
+            cellValues,
+            pointValues,
 
             mesh_.cellCentres(),
             mesh_.points(),