BUG: snappyHexMesh: patch intersections of mesh was using cell zone information.

This was going wrong in a fair few cases where you would get regions which
were delimited by a combination of intersections with unnamed surfaces (wall)
and named surfaces (faceZone surface) but not with either of these sets
individually. This would cause there to be unvisited cells (zone = -2)
next to a visited cells but separated by a named surface only (and not an
unnamed surface). Fixed by using direct surface intersection in the removal
part. Related to #66.