Design a faster ghost level detection for unstructured grids.

The previous algorithm was fairly naive. The new implementation
is signficantly faster when you only have a single ghost level,
and decently faster for ghost levels past one.

Change-Id: I7fa3bd1a54f22169f272139d6aa7530d43015d7e
Implement a faster ghost level subsetting for polydata.

Change-Id: I32c98b175922d014f0c837466e394dd40e69b9a9
Merge topic 'faster_extract_piece' into master

7ceb52f2 Implement a faster ghost level subsetting for polydata.
22c622b0 Design a faster ghost level detection for unstructured grids.