Split up nextgens

[HyperCodec]

I believe that implementing more complex environments (such as arena elimination) would be easier if nextgens are split into two distinct phases: Elimination and Repopulation.

• Current algorithms like crossover, division, etc. would be implemented as Repopulators.
• The pruning part would be implemented as Eliminator, with the current default one used in all pruning functions being called FitnessEliminator or something of the sort.
• Fitness functions would no longer be a part of GeneticSim, but rather FitnessEliminator. GeneticSim would accept an eliminator and a repopulator.

In the future, we can then implement things like ArenaEliminator for more complex simulations involving interactions between agents.

[HyperCodec]

merged in #96