Add instant playing function

[shshsunny]

Hi Nathan! I'm learning about neural network and genetic algorithm. I found your project which is not complicated but very helpful and educational. I tried to read and run your prgram. The simulation was great, but the frames could only be saved separately. I thought it'd be more convenient for learners to see the organisms' motion if display the frames instantly on the screen. It may not be very meaningful for serious research, but it can enrich the simulator's presenting form. The modified prgram has a new setting item "plot_pygame". It'll enable instant display function if is set to True. This function depends on the graphics lib pygame. I added another module plotting_pygame, similar to the module plotting. However, pygame and plotting_pygame will not be imported if "plot_pygame" item is set to False, so the new function will not disturb the other parts of the program. I tried to code in your code style, but I maybe still uglified it. I'm sorry. A learner really likes this project Sunny