Add flatten_array solution for U3

u3/flatten_array.py

@@ -0,0 +1,10 @@
+def flatten(iterable):
+    pusti = []
+    for gns in iterable:
+        if gns is None:
+            continue
+        elif isinstance(gns, (list, tuple)):
+            pusti.extend(flatten(gns))
+        else:
+            pusti.append(gns)
+    return pusti