Support elitism in multi_population_ea

[sujeet100]

Currently multi_population_ea() doesn't support elitism. One cannot just pass in the elitism operator since it needs access to the parents, which the caller doesn't have.

We can fix this by adding an elites parameter that specifies how many elites we want. If it's zero, we just fall back on the current behavior.
similar to #112

Happy to raise PR.