Adds a Visualizer for the Gravity Generator (#2277)

* Adds Visualizer to GravityGenerator

* nullable

* snake bad

* -Reads layers from yaml
-Reads sprite states from yaml

* Fix GravityGridTest

Content.Server/GameObjects/EntitySystems/GravitySystem.cs

@@ -1,7 +1,8 @@
-using System.Collections.Generic;
+using System.Collections.Generic;
 using System.Linq;
 using Content.Server.GameObjects.Components.Gravity;
 using Content.Server.GameObjects.Components.Mobs;
+using Content.Shared.GameObjects.Components.Gravity;
 using JetBrains.Annotations;
 using Robust.Server.GameObjects.EntitySystems;
 using Robust.Server.Interfaces.Player;