Is `src/gen_julia_matlab_code.jl` obsolete?

[mfasi]

I think that the code in src/gen_julia_matlab_code.jl is superseded by src/gen_code.jl, which is more complete and provides the same functionalities. If so, perhaps we should delete the former.

[jarlebring]

Yes. Go ahead.

While we are touching these files: I think the files for the code generators can be better structured. Let's create a directory code_gen and separate (as good as possible) code_gen/matlab_gen.jl, code_gen/julia_gen.jl, code_gen/c_gen.jl.