ENH: snappyHexMesh: multi-stage layer addition, automatic hole closure

Automatic hole closure:
- introduces 'holeToFace' topoSet source
- used when detecting a 'leak-path'
- creates additional baffles to close the leak

Multi-stage layer addition:
- Can add layers in multiple passes

See issues: #2403, #2404

tutorials/mesh/snappyHexMesh/sphere_gapClosure/system/controlDict

@@ -0,0 +1,54 @@
+/*--------------------------------*- C++ -*----------------------------------*\
+| =========                 |                                                 |
+| \\      /  F ield         | OpenFOAM: The Open Source CFD Toolbox           |
+|  \\    /   O peration     | Version:  v1906                                 |
+|   \\  /    A nd           | Web:      www.OpenFOAM.com                      |
+|    \\/     M anipulation  |                                                 |
+\*---------------------------------------------------------------------------*/
+FoamFile
+{
+    version     2.0;
+    format      ascii;
+    class       dictionary;
+    location    "system";
+    object      controlDict;
+}
+// * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * //
+
+//DebugSwitches
+//{
+//    holeToFace  1;
+//}
+
+application     icoFoam;
+
+startFrom       startTime;
+
+startTime       0;
+
+stopAt          endTime;
+
+endTime         100;
+
+deltaT          1;
+
+writeControl    timeStep;
+
+writeInterval   1;
+
+purgeWrite      0;
+
+writeFormat     ascii;
+
+writePrecision  6;
+
+writeCompression off;
+
+timeFormat      general;
+
+timePrecision   6;
+
+runTimeModifiable true;
+
+
+// ************************************************************************* //