Fix Snapshot Load and Save Functions

Were just pointers to the map before. Fix to be actual clones of the map.

lib/src/Simulation.dart

@@ -154,6 +154,10 @@ class Simulation {
     _dirty = true;
   }
 
-  void saveSnapshot() => _snapshot = map;
-  Grid<bool> loadSnapshot() => map = _snapshot;
+  void saveSnapshot() => _snapshot = Grid.from(map);
+  Grid<bool> loadSnapshot() {
+    map = Grid.from(_snapshot);
+    _dirty = true;
+    return map;
+  }
 }