ENH: snappyHexMesh: add buffer layers before snapping

Summary

Adding buffers layers before adding layers.

Details of new models (If applicable)

snappyHexMeshDict has a top-level entry describing the type of meshing:

// Type of mesh generation:
//  - castellated (default)
//  - castellatedBufferLayer
type            castellatedBufferLayer;

See e.g. tutorial mesh/snappyHexMesh/rotated_block

Risks

With default setting there should be no change in the mesh generation. Only if the face-splitting during snapping (nFaceSplitInterval) is used are there slight differences due to a fix in the logic.

applications/test/syncTools/missing_cell/constant/transportProperties

+/*--------------------------------*- C++ -*----------------------------------*\
+| =========                 |                                                 |
+| \\      /  F ield         | OpenFOAM: The Open Source CFD Toolbox           |
+|  \\    /   O peration     | Version:  v2312                                 |
+|   \\  /    A nd           | Website:  www.openfoam.com                      |
+|    \\/     M anipulation  |                                                 |
+\*---------------------------------------------------------------------------*/
+FoamFile
+{
+    version     2.0;
+    format      ascii;
+    class       dictionary;
+    location    "constant";
+    object      transportProperties;
+}
+// * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * //
+
+nu              0.01;
+
+
+// ************************************************************************* //