Updating vtkPythonExtractSelection (and extract_selection module) to use
the new (and improved) Python/VTK/numpy API. The code now relies heavily
on the Python calculator to do the heavy lifting. Once the  mask array
has been generated vtkPythonExtractSelection/extract_selection extracts
the selected elements, as requested.

Change-Id: I5938b55128353047375e825c1eea5b2b033fc54e