split collisions into a separate project (#308)

Source/MonoGame.Extended.Collisions/CollisionGridCell.cs

@@ -0,0 +1,26 @@
+using MonoGame.Extended.Shapes;
+
+namespace MonoGame.Extended.Collisions
+{
+    public class CollisionGridCell : ICollidable
+    {
+        private readonly CollisionGrid _parentGrid;
+
+        public CollisionGridCell(CollisionGrid parentGrid, int column, int row, int data)
+        {
+            _parentGrid = parentGrid;
+            Column = column;
+            Row = row;
+            Data = data;
+            Flag = data == 0 ? CollisionGridCellFlag.Empty : CollisionGridCellFlag.Solid;
+        }
+
+        public int Column { get; }
+        public int Row { get; }
+        public int Data { get; private set; }
+        public object Tag { get; set; }
+        public CollisionGridCellFlag Flag { get; set; }
+
+        public RectangleF BoundingBox => _parentGrid.GetCellRectangle(Column, Row).ToRectangleF();
+    }
+}