Spdhg with stochastic sampler

[MargaretDuff]

Describe your changes

• Allow SPDHG to take a sampler either from our sampler class or any class with a next(self) function
• Deprecated prob from SPDHG, taking the probabilities instead from the sampler class or from a new argument prob_weights, choosing the default [1/num_subsets]*num_subsets if one is not provided in either place.
• Created two setters for the step sizes. set_step_sizes_from_ratio resets the step sizes if the user provides one/both/none of gamma and rho - note that this closes SPDHG gamma parameter is applied incorrectly  #1860. step_sizes_custom takes in one/both/none of sigma and tau allowing the user to use a custom sigma and tau with those not provided calculated from the defaults. Calculating sigma from tau probably needs checking with someone else.
• Added a check_convergence function that checks self._sigma[i] * self._tau * self.norms[i]**2 <= self.prob_weights[i] for all i. This probably needs checking with someone else.
• Deprecated the kwarg "norms" to be replaced by the set_norms method in BlockOperator: added a function to return a list of norms and the ability to set this list of norms BlockOperator: added a function to return a list of norms and the ability to set this list of norms BlockOperator: added a function to return a list of norms and the ability to set this list of norms  #1513.
• Unit tests for SPDHG setters and convergence check

Describe any testing you have performed

Please add any demo scripts to CIL-Demos/misc/
Test with SPDHG https://github.com/TomographicImaging/CIL-Demos/blob/main/misc/testing_sampling_SPDHG.ipynb

Similar results gained for all samplers for SPDHG, with 10 subsets

With 80 subsets:

Link relevant issues

Part of the stochastic work plan. Closes #1575. Closes #1576. Closes #1500. Closes #1496

Checklist when you are ready to request a review

• I have performed a self-review of my code
• I have added docstrings in line with the guidance in the developer guide
• 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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• 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

[gfardell]

Mostly docstring clarification. I think it's very close.

[gfardell]

A couple of changes to the unit tests. But I'm happy for you to merge when you're happy!