Different genome initializers for different sub-populations in coevolution

[SigmaX]

Kexin wanted to use coevolution where individuals in each subpop have different lengths. But we take only one representation/initializer in multi_population_ea() and use it for all subpops.

Allow representation to take a list of Representations instead of only one Representation.

Bonus: this will allow us to support things like heterogeneous island models that have different Representations for each island.

• Update multi_population_ea()
• Add an example exercising the feature
• Unit test