Automatically update matplotlib transform involving `xy` coordinates when any disc parameters are changed

[ortk95]

Previously, the matplotlib transforms involving xy coordinates could end using old values if disc parameters were changed without also calling update_transform. Now, the transforms are automatically updated whenever any disc parameters are changed, ensuring that the transforms should always be consistent with the current disc parameters.

Also updated the documentation about the mutable nature of the transforms, and how to use their transform.frozen() method to freeze the transform if needed.

Closes #389

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.)
• [x] Check any changes to requirements.txt are reflected in setup.py and conda-forge feedstock
• [x] 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 10422301539

Details

• 9 of 9 (100.0%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 99.615%

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Totals
Change from base Build 10268963205:	0.0%
Covered Lines:	3138
Relevant Lines:	3150

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💛 - Coveralls