[General] Objective getter

[ocots]

Add a getter of objective for JuMP.

[amontoison]

@ocots Is it not easier for you to rely on NLPModelsJuMP.jl?
It transforms a JuMP model into a MathOptNLPModel where you can use all the API of NLPModels.jl.

-> https://jso.dev/NLPModelsJuMP.jl/dev/tutorial/

[ocots]

@amontoison I want to solve directly with JuMP so it is ok for me. But I can add a word about this possibility.