This commit introduces the vtkStructuredImplicitConnectivity class, intended
as a lower-level helper that provides functionality for resolving
situations where "gaps" are present in a distributed structured dataset. Gaps
are resolved by communicating the data (growing the grid to the right) as
necessary to close the gap. This situation is common with finite difference
grids that are partitioned with respect to the nodes of the grid, or when some
other post-processing filter, e.g., sampling the grid to reduce the resolution
introduces a gap.The present implementation requires one grid per process and
can deal with 2-D datasets -- XY,XZ,YZ -- as well as 3-D datasets. A test for
all 2-D and 3-D cases is also added.

Change-Id: I5260ce4fb5ced45434a719c8aacc2153253917a7