ENH: distinguish between cell/face/point topoSetSource (#1060)

- add intermediate classes topoSetCellSource, topoSetFaceSource,
  topoSetPointSource and corresponding New() factories

src/meshTools/sets/cellSources/fieldToCell/fieldToCell.C

@@ -38,6 +38,22 @@ namespace Foam
     defineTypeNameAndDebug(fieldToCell, 0);
     addToRunTimeSelectionTable(topoSetSource, fieldToCell, word);
     addToRunTimeSelectionTable(topoSetSource, fieldToCell, istream);
+    addToRunTimeSelectionTable(topoSetCellSource, fieldToCell, word);
+    addToRunTimeSelectionTable(topoSetCellSource, fieldToCell, istream);
+    addNamedToRunTimeSelectionTable
+    (
+        topoSetCellSource,
+        fieldToCell,
+        word,
+        field
+    );
+    addNamedToRunTimeSelectionTable
+    (
+        topoSetCellSource,
+        fieldToCell,
+        istream,
+        field
+    );
 }
 
 
@@ -100,7 +116,7 @@ Foam::fieldToCell::fieldToCell
     const scalar max
 )
 :
-    topoSetSource(mesh),
+    topoSetCellSource(mesh),
     fieldName_(fieldName),
     min_(min),
     max_(max)
@@ -113,10 +129,13 @@ Foam::fieldToCell::fieldToCell
     const dictionary& dict
 )
 :
-    topoSetSource(mesh),
-    fieldName_(dict.get<word>("field")),
-    min_(dict.get<scalar>("min")),
-    max_(dict.get<scalar>("max"))
+    fieldToCell
+    (
+        mesh,
+        dict.get<word>("field"),
+        dict.get<scalar>("min"),
+        dict.get<scalar>("max")
+    )
 {}
 
 
@@ -126,7 +145,7 @@ Foam::fieldToCell::fieldToCell
     Istream& is
 )
 :
-    topoSetSource(mesh),
+    topoSetCellSource(mesh),
     fieldName_(checkIs(is)),
     min_(readScalar(checkIs(is))),
     max_(readScalar(checkIs(is)))