Add transform_coords abstract method. SimplexTransform drops last coo…

[AndrewKillion]

Description

• Add abstract transform_coords method to Transform base class
• Implement method in all concrete Transform classes
• SimplexTransform and SumTo1 remove last coordinate
• Most other transforms preserve coordinates
• Add comprehensive test suite

Related Issue

• [ ] Closes #
• [X] Related to #7907

Checklist

• [x] Checked that the pre-commit linting/style checks pass
• [ ] Included tests that prove the fix is effective or that the new feature works
• [x] Added necessary documentation (docstrings and/or example notebooks)
• [ ] If you are a pro: each commit corresponds to a relevant logical change

Type of change

• [x] New feature / enhancement
• [ ] Bug fix
• [ ] Documentation
• [ ] Maintenance
• [ ] Other (please specify):

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📚 Documentation preview 📚: https://pymc--7913.org.readthedocs.build/en/7913/