Big commit!  (1) see_fish.m models the visual system of either fish. (2) I moved wall avoidance into simple_model.m. (3) animate_sim.m code moved into vis_results.m. (4) vis_instant.m added to plot the predator and prey for a moment of the simulation. (5) foraging.m replaced foraging_pred.m.  (6) I changed some of the parameter names in the 'p' structure of simple_model.m to match that of 'run_sim.m' so that functions can find those field whether during a simulation or during post-processing.

At this point, this code still does nothing with the prey.  The predator switches between wall avoidance, prey targeting, and saccades. However, with current parameters, few of the turns appear to have much to do with saccades.