fix 2D origin

[gfardell]

Describe your changes

Fix to show_geometry for 2D datasets with panel origin set.

Describe any testing you have performed

Please add any demo scripts to CIL-Demos/misc/
Checked origin marker for 2D/3D Cone/Parallel geometries.

Link relevant issues

closes #1491

Checklist when you are ready to request a review

• I have performed a self-review of my code
• I have added docstrings in line with the guidance in the developer guide
• I have implemented unit tests that cover any new or modified functionality
• CHANGELOG.md has been updated with any functionality change
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• Change pull request label to 'Waiting for review'

Contribution Notes

Please read and adhere to the developer guide and local patterns and conventions.

• The content of this Pull Request (the Contribution) is intentionally submitted for inclusion in CIL (the Work) under the terms and conditions of the Apache-2.0 License.
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[gfardell]

@lauramurgatroyd I think this addresses an issue you'd previously found when you tried to show the centre slice in a notebook. But I don't recall which one, do you?

[lauramurgatroyd]

I don't remember, sorry