dataexamples tidy

[casperdcl]

Changes

OOP tidy of #1712

• [ ] rebase after #1774
• drop unused imports
• added abc.ABC -> BaseTestData -> TestData,NexusTestData,RemoteTestData
• dropped some **kwargs (#1115)
• drop _download_and_extract_from_url
• use class properties more
• prefix underscore on private/protected members

Testing you performed

Related issues/links

• #1712
• #1115

Checklist

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

Contribution Notes

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

• [x] 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
• [x] I confirm that the contribution does not violate any intellectual property rights of third parties