Some concerns about our neural networks and Ctrnn

[Ploppz]

Organism and Gene

• In mutation_add_connection, there is no check whether the connection already exists. In Organism::get_weights, we only use the weight of one connection, so any other similar connections are useless.
• Bias can only be positive. I'm not sure if this is good? I also noticed in Ctrnn that we take the negative bias, rather than the positive as in the paper. So practically we always have a negative bias.
• What about having both bias: f64 and weight: f64 in Gene (instead of having a bias and a weight type of gene)?

Ctrnn

• Shouldn't tau be bigger than 1.0? I think it sort of represents how fast y decays. And when it is 1.0, y decays in only one timestep. (you can verify this by looking at how we calculate the new y (however look at my commit where I fixed a small thing in Ctrnn)