Step size property consistent across APGD, FISTA, ISTA, GD...

[MargaretDuff]

Changes

I tried to make the step size property consistent across APGD, FISTA, ISTA, and GD. This step size getter should no longer increment the step size. Updated the tests. Added reference to Armijo rule in GD docstring.

Testing you performed

Please add any demo scripts to https://github.com/TomographicImaging/CIL-Demos/tree/main/misc

Related issues/links

Closes #2151 and closes #2122

Checklist

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

Contribution Notes

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

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

[MargaretDuff]

Discussed in a CIL dev meeting this morning. Possibly needs a discussion about a parent class for first-order gradient descent methods: GD, ISTA/FISTA, APGD, to prevent duplication of this code