Add option to choose between planetographic and planetocentric coordinates when plotting gridlines in the GUI

[ortk95]

Added an option to the 'Edit' dialog for the gridlines in the GUI to allow the choice between planetographic and planetocentric coordinates. This means that it is now possible to create a plot using a planetocentric grid entirely using the GUI.

This is effectively adds the code changes made in #366 to the GUI. Along with the changes in #392, this brings full parity between planetographic and planetocentric coordinates in the GUI.

Closes #394

Pull request checklist

• Add a clear description of the change
• Add any new tests needed
• 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 10469982598

Details

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 99.617%

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Totals
Change from base Build 10469569340:	0.0%
Covered Lines:	3157
Relevant Lines:	3169

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💛 - Coveralls