Added the ability to specify a clip tolerance.

The clip tolerance can be set now. Default is FLT_EPSILON. Setting to too
large a value will cause numerical problems.

Change-Id: Icc332801ed4b9f00b6b6c58cb78b0a36186f4729
Merge topic '0013436-vtkBandedPolyDataContourFilter-ClipTolerance' into master

b04dd36c Added the ability to specify a clip tolerance.