By using the dual of the cellset we can quickly compute the GhostCells
of structured data using WorkletPointNeighborhood boundary condition
object
Using a 1024x1024x512 test grid we see the following perf:
Master Serial : 5.658144 sec
This MR Serial: 0.519684 sec
Master OpenMP : 0.532256 sec
This MR OpenMP: 0.080604 sec
Previously, the CellClassification enum was stored in a header file
named GhostCell.h, which made it hard to find and obscured its purpose.
Moved it to an appropriately named file. Also, renamed the DUPLICATE
field to GHOST to make its intention more clear.