ENH: fvMeshSubset improvements (issue #951)

- what was previously termed 'setLargeCellSubset()' is now simply
  'setCellSubset()' and supports memory efficient interfaces.

  The new parameter ordering avoids ambiguities caused by default
  parameters.

  Old parameter order:

      setLargeCellSubset
      (
          const labelList& region,
          const label currentRegion,
          const label patchID = -1,
          const bool syncCouples = true
      );

  New parameter order:

      setCellSubset
      (
          const label regioni,
          const labelUList& regions,
          const label patchID = -1,
          const bool syncCouples = true
      );

   And without ambiguity:

      setCellSubset
      (
          const labelUList& selectedCells,
          const label patchID = -1,
          const bool syncCouples = true
      );

- support bitSet directly for specifying the selectedCells for
  memory efficiency and ease of use.

- Additional constructors to perform setCellSubset() immediately,
  which simplifies coding.

  For example,

      meshParts.set
      (
          zonei,
          new fvMeshSubset(mesh, selectedCells)
      );

  Or even

      return autoPtr<fvMeshSubset>::New(mesh, selectedCells);

src/sampling/sampledSurface/isoSurface/sampledIsoSurface.C

@@ -376,14 +376,16 @@ bool Foam::sampledIsoSurface::updateGeometry() const
                 << patches[exposedPatchi].name() << endl;
         }
 
+        // Remove old mesh if any
+        subMeshPtr_.clear();
         subMeshPtr_.reset
         (
-            new fvMeshSubset(fvm)
-        );
-        subMeshPtr_().setLargeCellSubset
-        (
-            labelHashSet(mesh().cellZones()[zoneID_.index()]),
-            exposedPatchi
+            new fvMeshSubset
+            (
+                fvm,
+                mesh().cellZones()[zoneID_.index()],
+                exposedPatchi
+            )
         );
     }
 

src/sampling/sampledSurface/sampledCuttingPlane/sampledCuttingPlane.C

@@ -78,12 +78,12 @@ void Foam::sampledCuttingPlane::createGeometry()
 
         subMeshPtr_.reset
         (
-            new fvMeshSubset(static_cast<const fvMesh&>(mesh()))
-        );
-        subMeshPtr_().setLargeCellSubset
-        (
-            labelHashSet(mesh().cellZones()[zoneID_.index()]),
-            exposedPatchi
+            new fvMeshSubset
+            (
+                static_cast<const fvMesh&>(mesh()),
+                mesh().cellZones()[zoneID_.index()],
+                exposedPatchi
+            )
         );
     }