* Added centered unit box static field to Box2.
* MapGrid is more testable.
* Added some unit tests for MapGrid.
Fixed bug in MapChunk.GridTileToLocal().
MapGrid.UpdateAABB() actually expands properly now.
* Moved IMapChunk to Robust.Shared.Map.
Moved Chunk class out of MapManager class.
* Added unit tests for MapChunk.
* Bounds reduce by 1, so almost working.
* Now bound shrinking works :D
* Moved MapGrid out of MapManager.
Moved IMapGrid into the Shared/Map folder.
Replaced all calls to IMapGrid.ParentMap.Index with IMapGrid.ParentMapId.
* Added more MapGrid unit tests.
Fixed a bug in TryGetTileRef.
