StimFrame: Store ringbuff index as member var

Now each StimFrame knows its index within its parent SpMcRingbuff
object.

include/user/spMcRingBuffer.h

@@ -61,7 +61,7 @@ public:
 		{
 			slots[i].~StimulusFrame();  // Destroy default-constructed object
 			new (&slots[i]) StimulusFrame(
-				frameAssemblyDesc->slots[i], callbacks, flags);
+				frameAssemblyDesc->slots[i], callbacks, flags, i);
 		}
 	}