SIRT objective

[MargaretDuff]

Changes

• Changed the SIRT objective_function to match the mathematics - it is now a weighted least squares calculation i.e. $0.5*(Ax-b)^M(Ax-b)$ were $M=\frac{1}{A*\mathbb{1}}$.
• Updated the documentation to explain this
• Changed some of our SIRT tests so they do not all use the identity

We might need to consider the efficiency of the objective calculation

Testing you performed

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

Related issues/links

Closes #1787

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
• [ ] 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

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