Document, typehint, and rearrange `plugins/astra` classes

[WYVERN2742]

This is part of the software sustainability hackathon, and is part of a larger project to improve cil's documentation. #1683

Describe your changes

Since the changes are quite large, this PR is limited to the cil.plugins.astra folder.

No functional code changes were performed.

• Sorted imports
• Document and typehint functions, following the numpy standard
  • Private functions are not documented.
• Move __init__ to top of class
• Move private functions to bottom of class
• Standardise using " for strings
• Explicitly state classes as abstract classes

Describe any testing you have performed

Please add any demo scripts to CIL-Demos/misc/

Link relevant issues

• https://github.com/TomographicImaging/CIL/issues/1683

Checklist when you are ready to request a review

• [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 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