SAG and SAGA approximate gradient methods

[MargaretDuff]

Describe your changes

SAG and SAGA approximate gradient methods

Describe any testing you have performed

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

Link relevant issues

Closes #1342

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
• [ ] 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

[MargaretDuff]

Any thoughts on this wording?

[MargaretDuff]

https://github.com/TomographicImaging/CIL/pull/1624#discussion_r1671008081

Thanks @jakobsj - In regards to this comment, I have added documentation explaining mathematically what approximation is taken at each iteration and thus SAG and SAGA is more easily comparable

[MargaretDuff]

Thanks @paskino for getting this over the line!