This commit introduces new filters, vtkPExtractGrid, vtkPExtractVOI
and vtkPExtractRectilinearGrid. Each filter, extends the corresponding
serial filter, i.e., vtkExtractGrid, vtkExtractVOI and
vtkExtractRectilinearGrid, respectively. The parallel versions of the
filter add proper handling for distributed datasets. When sub-sampling,
a gap across partitions may be introducted. The filters use the
vtkStructuredImplicitConnectivity class to communicate the data across
process boundaries and resolve the gap.

Change-Id: I2b2a5ecd5841adceee4e30aecd30d485ecfff5df