code: public Chromosome<G> getChromosome() {
  		assert _chromosomes != null;
  		assert _chromosomes.get(0) != null;
  
  		return _chromosomes.get(0);
  	}
docs: Return the first chromosome. This is a shortcut for
  <pre>{@code
  final Genotype<DoubleGene>; gt = ...
  final Chromosome<DoubleGene> chromosome = gt.getChromosome(0);
  }</pre>
  
  @return The first chromosome.