Add support for log-scaling for scalar opacity function.
This issue was created automatically from an original Mantis Issue. Further discussion may take place here.
ParaView support using log-scale for color mapping, but not for scalar mapping. We need to add support for that.
For that, we will need to add support for log-scaling to vtkPiecewiseFunction, then to vtkDiscretizableColorTransferFunction (for opacity, it already support for coloring). (Make sure to add VTK-side tests for this to ensure good coverage).
Finally, the opacity transfer function editor cannot really support editing using the UI (since the curves will no longer be linear in log-space). So we will simply put a text-box in it saying use the "advanced" editor to manually specify control points.