Add getter and setter methods for wrapper

!!Need testing

lib/src/Grid.dart

@@ -1,4 +1,5 @@
 import 'dart:core';
+import 'dart:math';
 
 import 'package:collection/collection.dart';
 
@@ -19,4 +20,24 @@ class Grid<E> extends DelegatingList<E> {
         width = w,
         height = h,
         super(l);
+
+  E elementAtPos(int x, int y) {
+    int i = toIndex(x, y);
+    if (i >= length) throw RangeError.index(i, this);
+    _internal[i];
+  }
+
+  void setElement(int x, int y, E value) {
+    int i = toIndex(x, y);
+    if (i >= length) throw RangeError.index(i, this);
+    _internal[i] = value;
+  }
+
+  E get(int x, int y) => elementAtPos(x, y);
+
+  void set(int x, int y, E value) => setElement(x, y, value);
+
+  int toIndex(int x, int y) => y * width + x;
+
+  Point<int> toCoords(int index) => Point<int>(index % width, index ~/ width);
 }