Modified vtkExtractCTHPart class to add the ability to extract
3D solid fragments as output instead of contours, using a boolean
flag (GenerateSolidGeometry). Output format, in that case, is
vkUnstructuredGrid. Default state is off for this flag.
Added a python test TestExtractCTHPartSolid.py to test
the new code branch of class vtkExtractCTHPart.