Move Simulation Accesses to Simulation Class

Everything has been refactored away from engine, which now only controls updating & rendering within a specific timestep. (As well as stepping forward by calling a single update)

Everything regarding grids, patterns and cells has been moved into the simulation and the Services have been updated to reflect that.

test/src/engine_test.dart

@@ -31,12 +31,4 @@ void main() {
       expect(sut.canvas, isNotNull);
     });
   });
-  group("gridSize", () {
-    test("zero gridSizes throw ArgumentErrors", () {
-      expect(() => sut.gridSize = Point(0, 5), throwsArgumentError);
-    });
-    test("negative gridSizes throw ArgumentErrors", () {
-      expect(() => sut.gridSize = Point(1, -5), throwsArgumentError);
-    });
-  });
 }