WIP: Python example testing

This adds a test driver script called "RunPythonTest.py" that can run a python example as a test:

python RunPythonTest.py -D Data src/Python/VisualizationAlgorithms/HeadBone.py

It introspects the test for a 'test_data' attribute. If present, this attribute gives the test files used by the example. Potentially, a more generic 'test_args' attribute could provide default args for testing purposes, and 'test_data' would simply indicate which args are data files.

test_data = ['FullHead.mhd']
test_args = [test_data[0], 1, 2]

Generation of regression test images is still on the to-do list. Currently, most tests call iren.Start() which provides interaction rather than non-interactive testing. Ideally, the test driver script should override the vtkRenderWindowInteractor so that Start() performs regression testing rather than initiating the interaction.