Improve vtkThreshold API
Restore convenience functions that were previously considered to be removed (see https://discourse.vtk.org/t/unnecessary-vtk-api-change/9929/4).
Add convenience methods for setting the threshold function.
Restore convenience functions that were previously considered to be removed (see https://discourse.vtk.org/t/unnecessary-vtk-api-change/9929/4).
Add convenience methods for setting the threshold function.