Implement IEnumerable for System groups

Author: JoNax97

Arch.System/Systems.cs

@@ -2,6 +2,7 @@
     #define ARCH_METRICS_DISABLED
 //#endif
 
+using System.Collections;
 using System.Diagnostics;
 using System.Runtime.CompilerServices;
 using System.Text;
@@ -85,7 +86,7 @@ public virtual void Dispose(){}
 ///     They will run in order.
 /// </summary>
 /// <typeparam name="T">The type passed to the <see cref="ISystem{T}"/>.</typeparam>
-public class Group<T> : ISystem<T>
+public class Group<T> : ISystem<T>, IEnumerable<ISystem<T>>
 {
 #if !ARCH_METRICS_DISABLED
     private readonly Meter _meter;
@@ -347,6 +348,21 @@ public override string ToString()
 
         return $"Group = {{ {nameof(Name)} = {Name}, Systems = {{ {stringBuilder} }} }} ";
     }
+    
+    /// <inheritdoc/>
+    public IEnumerator<ISystem<T>> GetEnumerator()
+    {
+        foreach (var entry in _systems)
+        {
+            yield return entry.System;
+        }
+    }
+
+    /// <inheritdoc/>
+    IEnumerator IEnumerable.GetEnumerator()
+    {
+        return GetEnumerator();
+    }
 
     /// <summary>
     ///     The struct <see cref="SystemEntry"/> represents the given <see cref="ISystem{T}"/> in the <see cref="Group{T}"/> with all its performance statistics.