Feature request: change size of constraints without manually re-creating the solver

[traversaro]

As discussed with @giulioromualdi, it would be great to be able to change the size of the constraints without having to manually recreate the solver.

We could add the feature of automatically re-creating the solver under the hood if a user change the size of the constraints. As this feature is not supported by the underlying qp solvers, there still be a relative huge cost in recreating the solver instance, but the main advantage would be that there would be no need for the caller to manually set again all the settings.

On the qpsolvers-eigen side, we would just need to be able to save all the settings, to be able to set them again once the number of constraints change.