What does the `plugin` mean in `nosnoc.solver.mpccsol` and how should it be passed.

[apozharski]

The current interface for mpccsol is designed to be similar to nlpsol i.e. mpccsol(solver_name, plugin, problem, options). In its current state we only support nosnoc.solver.MpccSolver as a backend for mpccsol which is a relaxation solver for MPCCs. This means plugin is used to pass the relaxation method to mpccsol. This is in my opinion problematic as it both makes changing the relaxation mode a bit unintuitive and will become more confusing as other solver backends are added.

We have as I see it several options:

1. Continue as is but then add other plugins.
2. Drop the plugin as a part of the interface and simply pass it as part of the options.
3. Change the plugin name for the current MpccSolver backend to something like relaxation (name open to change) and have the relaxation mode as part of the solver options.

I view 3 as the best option as it is relatively ergonomic and makes it clear which solver backend is being used. 2 is also passable for me though it moves away from the nlpsol interface which may be undesirable in terms of user ergonomics.