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.