Add option to customise scaling of wireframe plots

[ortk95]

Adds a new scale_factor argument to scale wireframe plots. This can be used to customise the units of the plotted coordinates, for example body.plot_wireframe_km(scale_factor=1000) will create a plot with units of metres rather than kilometres.

Closes #333.

Pull request checklist

• [x] Add a clear description of the change
• [x] Add any new tests needed
• [x] Run spell check on new text visible to user (documentation, GUI etc.)
• [ ] Check any changes to requirements.txt are reflected in setup.py and conda-forge feedstock
• [ ] Check code passes CI checks (run run_ci.sh or check GitHub Actions)

See CONTRIBUTING.md for more details.

[coveralls]

Pull Request Test Coverage Report for Build 7917423046

Details

• 15 of 15 (100.0%) changed or added relevant lines in 2 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.03%) to 99.571%

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Totals
Change from base Build 7873948293:	0.03%
Covered Lines:	2872
Relevant Lines:	2884

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💛 - Coveralls