Implement adding Hessian diagonal in SIRF

[evgueni-ovtchinnikov]

Changes in this pull request

Testing performed

Related issues

Fixes #1369

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[KrisThielemans]

I think the best strategy is to just use GibbsRelativeDifferencePenalty everywhere. It should be backwards compatible with RelativeDifferencePrior. @ColomboMatte0 is that correct? (we don't need the surrogate stuff here AFAIK)

[ColomboMatte0]

It is backwards compatible with all the methods implemented in RelativedifferencePrior , we do not have SPS stuff for the RDP.