CIL recon: adding implicit formulation of TV with warm start

[paskino]

Issue

closes #1971

Description

Adds the possibility to run TV regularised LS with the implicit formulation of (S)PDHG. This will require some UI changes, and in future it can be used to define different regularisers (like TGV), although the latter will require additional UI changes + the new dependency on the CCPi Regularisation.

Testing

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Acceptance Criteria

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Documentation

How have you changed the documentation to reflect your changes? All changes should be noted in the appropriate file in docs/release_notes

[samtygier-stfc]

Thanks, I'll take a look at this next week.