Parallel agents movement.

src/main.rs

@@ -1,6 +1,8 @@
 use physarum::model;
 
 fn main() {
-    let model = model::Model::new(4, 4, 20, 1);
+    let mut model = model::Model::new(4, 4, 20, 1);
+    println!("{:#?}", model);
+    model.step();
     println!("{:#?}", model);
 }