Implement L-BFGS

[David Tolpin]

Implement L-BFGS for estimation of MLE.

[David Tolpin]

In the meantime L-BFGS from gonum can be used, at the cost of computing the value twice (once for the value, once for the gradient). https://godoc.org/gonum.org/v1/gonum/optimize

example to follow

[David Tolpin]

infer.FuncGrad returns the model wrapped as a function and it's gradient suitable for fields of gonum optimize.Problem. With that, all excellent optimization algorithms in gonum, including L-BFGS but also others are available.