snappyHexMesh occasionally hangs in parallel layer extrusion

• case was hanging in parallel
• with one of the processors in an infinite loop (in addPatchCellLayer)

Problem:

• shm layer addition can get any mesh to add layers to.
• sometimes this mesh might have a baffle on the patch-to-extrude (- a baffle is two back-to-back faces that share the same points)
• this is of course not possible to extrude so we disable this

However

• what if the baffle faces are on different processors
• each processor does not detect that the face is part of a baffle
• so might try to extrude

and this would lead to a hang since all the edges of the face connect to the same neighbour (admittedly on a different processor).