BIG rewrite and changes (too many to list)

src/main.rs

@@ -11,8 +11,8 @@ fn main() {
     // let (width, height) = (1024, 1024);
 
     // # of agents
-    let n_particles = 1 << 10;
-    // let n_particles = 1 << 16;
+    // let n_particles = 1 << 10;
+    let n_particles = 1 << 16;
     println!("n_particles: {}", n_particles);
 
     let diffusivity = 1;