Make generated FMU source code easier to modify?

[KristofferSkare]

The process of building the ML-FMU can be split into two parts to enable editing of the source code using the commands codegen and compile instead of build. There is maybe a question of if this is really a necessary feature to have. But if we are keeping the feature I think it should be improved.

However, actually doing some useful modifications of the source code is quite difficult. This is because of two main reasons.

• Lacking documentation of the OnnxFmu class that the generated inherits from
• All member variables of OnnxFmu are private making access to them difficult or impossible in some cases.

In addition it might be hard for users to even know how or why one would do this. I (@KristofferSkare ) can think of some cases where it is useful.