callback argument support for Metaheuristics

[Richert]

Providing the cb keyword argument to a GalacticOptim.solve call that uses an optimizer from the Metaheuristics.jl package currently yields the warning 'Callback argument is currently not used by Metaheuristics.AbstractAlgorithm' and is ignored. However, Metaheuristics.optimize provides a keyword argument logger that could easily be used to provide any callback function to Metaheuristics optimizers that merely serve the purpose of visualizing the current status of the optimization process.

I thus suggest to provide the cb value to logger and display a warning instead that warns the user that Metaheuristics only supports callback functions for visualization purposes at the moment.