WIP: Augment vtkBandedPolyDataContourFilter to facilitate global contouring.

This fix is in reference to bug report 0015413. banded contours are only locally consistent (across multiple surfaces, the contours are not guaranteed to match). Also, zero-area polygons may be generated within contour bands. This patch adds a run-time option to use an algorithm that chains cells' clip and contour methods to create globally continuous banded contours. The new algorithm is much slower (~10x) than the preexisting one.