feat: allow to halt iterations based on callback return value

[bvdmitri]

This PR fixes #84. @John-Boik

After this PR it is possible to return a true/false value from the before_iteration/after_iteration callbacks. true value indicates that the inference procedure should be halted and no further inference should be performed.

The solution here is not ideal. Ideally we should support different "convergence" strategies, e.g based on BFE values. I like the proposal from @John-Boik in the #84, but I believe it must be a separate issue. For now I believe its fine to use callbacks that allow to halt iterations based on true/false.

[bvdmitri]

@albertpod ping

[codecov-commenter]

Codecov Report

Patch coverage: 91.30% and project coverage change: +0.40 :tada:

Comparison is base (8a89b44) 79.88% compared to head (94babb1) 80.28%.

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Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #85 +/- ## ========================================== + Coverage 79.88% 80.28% +0.40% ========================================== Files 11 11 Lines 1173 1182 +9 ========================================== + Hits 937 949 +12 + Misses 236 233 -3 ``` | [Impacted Files](https://codecov.io/gh/biaslab/RxInfer.jl/pull/85?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=biaslab) | Coverage Δ | | |---|---|---| | [src/score/actor.jl](https://codecov.io/gh/biaslab/RxInfer.jl/pull/85?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=biaslab#diff-c3JjL3Njb3JlL2FjdG9yLmps) | `83.33% <87.50%> (-0.99%)` | :arrow_down: | | [src/inference.jl](https://codecov.io/gh/biaslab/RxInfer.jl/pull/85?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=biaslab#diff-c3JjL2luZmVyZW5jZS5qbA==) | `75.76% <92.30%> (+0.96%)` | :arrow_up: | | [src/helpers.jl](https://codecov.io/gh/biaslab/RxInfer.jl/pull/85?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=biaslab#diff-c3JjL2hlbHBlcnMuamw=) | `100.00% <100.00%> (ø)` | | Help us with your feedback. Take ten seconds to tell us [how you rate us](https://about.codecov.io/nps?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=biaslab). Have a feature suggestion? [Share it here.](https://app.codecov.io/gh/feedback/?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=biaslab)

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[bvdmitri]

The before/after_iteration existed before and their purpose is not necessarily to halt the iterations. Previously, they only allowed to do some extra actions (e.g. logging execution), but now they also allow to halt.