Add counters to slack models

[tmigot]

For complete compatibility of all the NLPModels, shouldn't the SlackModel and the SlackNLSModel have a counters attribute? Typically, a JSO solver would use nlp.counters to verify limits on number of evaluations.

If you think that's useful, I can prepare a PR.

[dpo]

I think that would be useful, but they should redirect to counters of the underlying model.

[tmigot]

This is not needed, if we use the API to access evals.