Some fixes for RunPython

[llaniewski]

This PR:

• introduces an example xml case which uses RunPython to construct vtkImageData and write it to disk.
• fixes and error in RunR occurring when accessing .si quantities
• converting R factors (eg. components of geometry) to numpy arrays of integers, shifted by 1, to make it consistent with python indexing. Previously an array of strings was visible in python, which was very inefficient. (FYI: @shkodm)

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (36c4007) 44.46% compared to head (614f37d) 44.46%.

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Additional details and impacted files

@@           Coverage Diff           @@
##           master     #485   +/-   ##
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  Coverage   44.46%   44.46%           
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  Files         166      166           
  Lines        7536     7536           
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  Hits         3351     3351           
  Misses       4185     4185           

Flag	Coverage Δ
d2q9	31.92% <ø> (ø)
d3q27_PSM_NEBB	36.53% <ø> (ø)
d3q27_pf_velocity	30.91% <ø> (ø)

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