ENH: RectilinearGrid implicit connectivity support

Added support for vtkRectilinearGrid type in the structured
implicity connectivity class. Also, extended the test to
check for correctness of the new functionality. Further,
updated RectilinearGrid partitioner such that it can either leave a
gap between partitions or not. Also, refactored and improved the
implementation.

Change-Id: I658e2798bd9720a7baafd7eb1eed3f1e8befafe0