More Pythonic VTK wrapping

[toddy (Todd)]

@berk.geveci

output_data = aReader.execute()
# vs
aReader.Update()
output_data = aReader.GetOutput()

output_data = aFilter.execute(input_data)
#vs
aFilter.SetInputData(input_data)
aFilter.Update()
output_data = aFilter.GetOutput()

To my mind execute() doesn’t return anything. Methods build() or generate() would be better.

Alternatively a breaking change to
aReader.GetOutput(bool update = false)
or
aFilter.GetOutput(vtkDataObject* data = nullptr)