Add Tolerance for vtkPolygon's and vtkPolyhedron's IsConvex() Method

Spiros Tsalikisrequested to merge
spiros.tsalikis/vtk:poly-gon-hedron-isconvex into master

In VTK, the IsConvex() method of vtkPolygon and vtkPolyhedron classes is used to determine if the polygon or polyhedron is convex. By default, the tolerance is DBL_EPSILON, which sometimes may be too strict. That's why the IsConvex() method now has an additional parameter tolerance that allows the user to specify the tolerance for the convexity check.

Additionally, vtkCellValidator makes use of this new tolerance parameter to check if the cell is convex using its Tolerance member variable.

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