add support for simple aggregation group entity

Sia/Views/Aggregator/AggregationGroup.cs

@@ -0,0 +1,10 @@
+namespace Sia;
+
+public record struct AggregationGroup<TId> : IAggregationEntity<TId>
+    where TId : IEquatable<TId>
+{
+    public Aggregation<TId> Aggregation { get; set; }
+
+    public static EntityRef Create(World world)
+        => world.GetHashHost<AggregationGroup<TId>>().Create();
+}

Sia/Views/Aggregator/Aggregator.cs

@@ -116,4 +116,9 @@ private void RemoveFromAggregation(in EntityRef entity, in TId id)
             aggrEntity.Dispose();
         }
     }
+}
+
+public class Aggregator<TId> : Aggregator<AggregationGroup<TId>, TId>
+    where TId : IEquatable<TId>
+{
 }