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