Matlab interface overhaul to match the Python interface in solver creation

Short update on this:
Matlab interface overhaul by FreyJo · Pull Request #1193 · acados/acados · GitHub is merged and the changes announced here are now in the acados master branch and will be in the next release, i.e. the one after Release v0.3.6 · acados/acados · GitHub
Feedback is still highly appreciated!