Allow async fitness functions

[jenetics]

The fitness function currently must be evaluated synchronous. It should be possible to define fitness functions which return a Future object.

public Future<Double> fitness(final Genotype<DoubleGene> gt) {
    return ...;
}

Genotype<DoubleGene> gtf = Genotype.of(DoubleChromosome.of(0, 1));
Engine<DoubleGene, Double> engine = Engine.build(this::fitness, gtf).build();

[gfrison]

Hello,

Instead of Future could be possible to return a CompletableFuture? The latter is more suitable for callbacks.

Thank you,

[jenetics]

Merged into r5.0.0 branch.