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.IntegrationTests/Tests/GravityGridTest.cs

@@ -1,6 +1,7 @@
 using System.Threading.Tasks;
 using Content.Server.GameObjects.Components.Gravity;
 using Content.Server.GameObjects.Components.Power.ApcNetComponents;
+using Content.Shared.GameObjects.Components.Gravity;
 using Content.Shared.Utility;
 using NUnit.Framework;
 using Robust.Shared.Interfaces.GameObjects;