Placeholder for separation on div-0 error

This calculation should produce an infinity, or NaN. Either way, it's an
error that definitely still exists. I'll make a test for it, and then
fix it at some point. This is so that I don't forget.

src/birdoids_plugin.rs

@@ -445,6 +445,12 @@ mod tests{
         );
     }
 
+    // Separation 0,0 test
+    #[test]
+    fn check_separation_zero_zero() {
+        todo!("Check what happens when a boid is on top of the point it's trying to move away from");
+    }
+
     // *********************
     // Separation x-axis tests
     // *********************