Fix the vtkPolyPlane implicit function

It now handles boundary conditions correctly.

Change-Id: I135347c583b1748cd3a298f06a2974953afc7c2b
Change vtkProbeFilter to expose the 'in' threshold

The error threshold for when a point is considered 'in' a cell can now
be either computed as it was (default) or manually set on the probe
filter.

Change-Id: I6f9481615cc868f882f0bcba3199182e453557c4
Merge topic 'slice_along_polyline' into master

ad5d35cd Change vtkProbeFilter to expose the 'in' threshold
ced07563 Fix the vtkPolyPlane implicit function