ENH: NearlyEqual fuzzy comparator and SafeDivision

Added a "NearlyEqual" fuzzy comparator based on Knuth,
"The Art of Computer Programming (vol II)" and safe division
that catches overflow/underflow.

Change-Id: Iec61417c59936831a44f8961eeaa7d44c40bee64
ENH: Ghost zone generation for unstructured grids

This commit adds functionality for generating ghost zones
on a distributed unstructured grid.

Change-Id: Idd9ce355f8070bbdf882cdb2350fdcbce2c4b845