Create an example for PyImageFilter

[thewtex]

https://github.com/InsightSoftwareConsortium/ITK/blob/master/Wrapping/Generators/Python/PyUtils/itkPyImageFilter.h

CC: @fbudin69500

[jeffduda]

Are there any "unofficial" examples of how to use this filter. I've been trying to use it. I can get a passed python function to get called, but it's unclear (to me) how I can access the input image or set the output image of the filter. https://discourse.itk.org/t/how-to-use-pyimagefilter/4593

[thewtex]

@jeffduda please review this PR: InsightSoftwareConsortium/ITK#3131