[TODO] topology defaulting API

[HyperCodec]

Currently, nets are initialized by linking all output nodes with all inputs and applying sigmoid to all outputs. Instead of this, maybe could introduce some sort of "network constructor" trait that lets certain structs init a neural network (ex: SigmoidMLP::new<I, O>() -> NeuralNetworkTopology<I, O> or something of the sort).

[HyperCodec]

Perhaps it would be better to make NeuralNetworkTopology::new require a generic constructor to simplify the API. (ex: NeuralNetworkTopology<1, 2>::new(SigmoidMLP::default(), mutation_rate, mutation_passes))