avoid copy in flatten by not calling concatenate

Author: alexfikl

arraycontext/container/traversal.py

@@ -631,7 +631,10 @@ def _flatten(subary: ArrayOrContainerT) -> None:
 
     _flatten(ary)
 
-    return actx.np.concatenate(result)
+    if len(result) == 1:
+        return result[0]
+    else:
+        return actx.np.concatenate(result)
 
 
 def unflatten(