Refactor vtkExtractCTHPart.

Cleanup vtkExtractCTHPart to use VTK 6.0 enhancements. Also exposed API to
control removal of ghost cells/capping/triangulation. Not removing ghost-cells
overcomes holes inherent in the algorithm employed.

Change-Id: I61f3abd4dc93067e4bdc7921d7247e033814e11a
Merge topic 'fix_extract_cth_filter' into master

323ad383 Refactor vtkExtractCTHPart.