Allow TV proximal to be calculated in place

[MargaretDuff]

In place = func.proximal(x, tau, out=x)

Changes

• I don't think there is any way round duplicating memory i.e. using TV prox in-place does not reduce memory compared to not using it in place. This is because in each inner iteration, the value of $x$ is required and thus needs to remain unchanged. So, in the case of an in-place call, I added a new x_eval, which does not share memory with out in the code.
• I made sure our unit tests were checking the strongly convex case

Testing you performed

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

Related issues/links

Closes #1650

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

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