Parallelize genetic programming based solver

[hidmic]

By moving away from plain NSGA-II, or perhaps by combining it with some niche or "island" strategy, it may be possible to distribute the search load across multiple cores. DEAP already has some examples using SCOOP. Pickling support for candidates is there.

[hidmic]

To offload fitness evaluation to a pool of workers is the simplest approach, and the one exercised by c67b2ce.