TotalGeneralisedVariation Function class

[epapoutsellis]

Describe your changes

Implements TotalGeneralisedVariation Function where the proximal method configures and runs the PDHG algorithm,i.e.

Using the definition of TGV, we have

And PDHG solves the latter problem, finding two minimisers (u,z).

• The call method of the TotalGeneralisedVariation class will be:

  

Describe any testing you have performed

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

Link relevant issues

closes #1038 closes #1122

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
• [x] 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.

[jakobsj]

Is this PR ready for review? If still in progress, please make draft.

[paskino]

closes #1038

[paskino]

This PR contains some backward incompatible changes to the CCPi regularisation toolkit plugin for TGV, which needs to be addressed.

WIP in https://github.com/paskino/CIL/tree/TotalGeneralisedVariation

For reference on the parameters of the TGV plugin, check https://github.com/TomographicImaging/CIL/pull/971#issuecomment-945660804