Implement proper Callback for NCC value

[roflmaostc]

https://discourse.julialang.org/t/optim-jl-callback/52625

[RainerHeintzmann]

I think that should have been handled in the pull request #33
but it uses the "x" value on the dictionary. But this works fine. I think the trick is to write
tr[end].metadata["x"] and tr[end].iteration, with tr being the argument to the callback.

[roflmaostc]

Yes, will close as soon as #33 is merged.