Introduce Bethe Free Energy

[ThijsvdLaar]

This branch includes an introduction to the Bethe Free Energy in the docs. Fixes #152 and #95.

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Comparison is base (cdadfb7) 81.03% compared to head (09b20fe) 81.03%. Report is 4 commits behind head on main.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #173 +/- ## ======================================= Coverage 81.03% 81.03% ======================================= Files 11 11 Lines 1271 1271 ======================================= Hits 1030 1030 Misses 241 241 ```

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[ThijsvdLaar]

Thanks for the feedback, I've made some changes to address your concerns.

@ismailsenoz might have more to say on these matters.

[bvdmitri]

If the user chooses a constraint involving variables that are not argument to common factors it will be ignored by the inference engine due to Bethe assumption. There is always implicit Bethe constraints that the user should always be aware of

I think this guide should not be a guide about @constraints, the guide about @constraints is a separate documentation page and we should keep in mind to mention it there https://github.com/biaslab/RxInfer.jl/issues/150

[ThijsvdLaar]

@bvdmitri I've added some of my go-to papers.

[bvdmitri]

Lets see if tests are still passing......😁