estimateGrindingSlicingPlanes is not optimal

[VictorLamoine]

When estimating the slicing planes we are detecting the mesh bounds in order to determine plane equations for the slicing.

We didn't find a clean solution to find just the right number of planes so we are adding (at least) 1 plane on each side of the middle plane to make sure we'll be slicing everything.

This is not a big problem as the slicing will return an empty vtkStripper and we'll just skip this plane but this may cause problems on some meshes.