ENH: snappyHexMesh: added to automatic gap-refinement capabilities

1. multi-ray shooting. It now shoots rays in all the 3 coordinate directions
from the cell centre. Before it would shoot just a single ray from the
nearest point on the surface, going through the cell centre.

There is a cost overhead in that now it shoots 6 rays (+-x, +-y, +-z)
instead of just 1.

2. bleeding of refinement. It marks the cells inside a gap and walks out
the gap-size to neighbouring cells (which are just outside the gap). This
should make for a smoother refinement pattern.