OEP: function-objects should keep track of their own call counts

[timholy]

(OEP = Optim Enhancement Proposal) Rather than keeping track of f_calls and g_calls in the algorithm, it seems much cleaner to have DifferentiableFunction store these quantities, and every time you call value(d, x) and gradient!(g, d, x) the counters get updated. That way there is no chance you're not keeping track of each evaluation (e.g., #219).

[pkofod]

I agree, and I know others do as well. We've had other suggestions for value and gradient!, but I actually like both.

[pkofod]

I'd throw my time after this asap (as in asap, not open source work asap), but (and correct me if I'm mistaken) I've seen @johnmyleswhite oppose something similar earlier. Any opinions? If I remember correctly, you said you wanted to go in the opposite direction API wise. Care to elaborate? Or correct me?