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 :white_check_mark:

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

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Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #485 +/- ## ======================================= Coverage 44.46% 44.46% ======================================= Files 166 166 Lines 7536 7536 ======================================= Hits 3351 3351 Misses 4185 4185 ``` | [Flag](https://app.codecov.io/gh/CFD-GO/TCLB/pull/485/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=CFD-GO) | Coverage Δ | | |---|---|---| | [d2q9](https://app.codecov.io/gh/CFD-GO/TCLB/pull/485/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=CFD-GO) | `31.92% <ø> (ø)` | | | [d3q27_PSM_NEBB](https://app.codecov.io/gh/CFD-GO/TCLB/pull/485/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=CFD-GO) | `36.53% <ø> (ø)` | | | [d3q27_pf_velocity](https://app.codecov.io/gh/CFD-GO/TCLB/pull/485/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=CFD-GO) | `30.91% <ø> (ø)` | | Flags with carried forward coverage won't be shown. [Click here](https://docs.codecov.io/docs/carryforward-flags?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=CFD-GO#carryforward-flags-in-the-pull-request-comment) to find out more.

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