Add Coordinates setter

Sets the corresponding element to the parameter value passed in. Checks against the grid size constraints beforehand and throws RangeError if outside of constraints. Preferred method to set
element via coordinates.

test/src/grid_test.dart

@@ -138,4 +138,30 @@ void main() {
       expect(sut.toCoordinates(6), equals(Point(0, 2)));
     }, tags: const ["happy"]);
   });
+  group("coordinates setter", () {
+    Grid<String> sut;
+    setUp(() {
+      sut = Grid(3, 3);
+      sut.setAll(0,
+          ["Hey", "you", "me", "together", "Hello", null, "I", "am", "ready."]);
+    });
+    test("sets element to null if passing null in", () {
+      sut.set(1, 1, null);
+      expect(sut.get(1, 1), null);
+    }, tags: const ["sad"]);
+    test("throws RangeError if setting element outside of grid width", () {
+      expect(() => sut.set(4, 1, "testValue"), throwsRangeError);
+    }, tags: const ["bad"]);
+    test("throws RangeError if setting element outside of grid height", () {
+      expect(() => sut.set(1, 4, "testValue"), throwsRangeError);
+    }, tags: const ["bad"]);
+    test("sets element at correct index", () {
+      sut.set(1, 0, "testValue");
+      expect(sut.get(1, 0), "testValue");
+    }, tags: const ["happy"]);
+    test("sets last element correctly", () {
+      sut.set(2, 2, "testValue");
+      expect(sut.get(2, 2), "testValue");
+    }, tags: const ["happy"]);
+  });
 }