Keep track of number of function / gradient evaluations

[mblondel]

Some methods, such as line-search based methods, require more function / gradient evaluations than others. It would be great to keep track of the number of such calls in the state. For instance, we could include state.num_fun_calls and state.num_grad_calls. This would allow to plot objective value as a function of these numbers and therefore compare various methods objectively.