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;
+  }
 }

test/simulation_test.dart

@@ -37,12 +37,14 @@ void main() {
     }, skip: "can not find a way to set dirty to true first yet");
   });
   group("save&load", () {
-    test("saves the current map, can be loaded by loadSnapshot", () {
-      var snapshot = sut.map;
+    test(
+        "saves a copy of the map which does not change when the actual map changes",
+        () {
       sut.saveSnapshot();
-      sut.map = MockGrid();
+      sut.mergeStateChanges({1: true, 2: true});
+      var snapshot = Grid.from(sut.map);
 
-      expect(sut.loadSnapshot(), equals(snapshot));
+      expect(sut.loadSnapshot(), isNot(equals(snapshot)));
     });
   });
 }