Add linear API

[tmigot]

See NLPModels.jl 0.18/0.19

I have a draft here https://github.com/JuliaSmoothOptimizers/AmplNLReader.jl/tree/add-linear-API but we probably should add functions to the C interface.

There might be an issue to fix with NLPModelsTest as well: JuliaSmoothOptimizers/NLPModelsTest.jl#54

[amontoison]

@tmigot Do you think that we can use your branch add-linear-API to fix the CI tests?

[tmigot]

Yes that's a good idea. I think we were waiting for somebody to grab this.

Do you need me to open the PR?

[amontoison]

Yes, I can continue it if you explain me what is missing.
Because the PR is quite old, I'm a little bit lost.

[tmigot]

No problem, I opened the PR and tried to add some comments. My PR was not ideal as it was only using the functions from Ampl API that were already exported. It might be that they have some tailored functions that we didn't have.