Refactor moi_function and jump_function into separate file

[odow]

x-ref #4189

We don't necessarily need to merge this. But these functions were spread all over the place. I'll comment in-line on something.

[odow]

We just do it explicitly here now instead.

[blegat]

If we're going to call check_belongs_to_model every time we call moi_function (which makes sense), we might as well just call it inside moi_function no ? Since moi_function takes 2 arguments, it makes sense to check that they model is indeed the correct one, doesn't it ?

[odow]

And now I have a much more sensible fallback.

[blegat]

I think the definition should be: The user should always call moi_function(model, f). When you implement, you can either implement moi_function(model, f) (and checking that the model match) or just implement moi_function(f).
To me, it makes sense to check that the model match precisely when we transform the JuMP function to MOI since that's when the variable references are going to be turned into indices (so without the model). Implementing it in two steps 1) check membership and then 2) do a moi_function(f) is one way to do it but they could also just be done together, which is what we do for NonlinearExpr

[odow]

We can't do that because that's then a breaking change. People can and have been calling moi_function(f).

[blegat]

It's the new way forward but we don't have to enforce it. It's for clarifying in the comments& doc

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.85%. Comparing base (19eb40e) to head (e037eb1).

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #4190      +/-   ##
==========================================
- Coverage   99.93%   99.85%   -0.08%     
==========================================
  Files          42       43       +1     
  Lines        6305     6307       +2     
==========================================
- Hits         6301     6298       -3     
- Misses          4        9       +5     

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[odow]

Okay @blegat how is this.

[blegat]

Looks good modulo my minor comments to improve readability