Add method next

Add method next, which was removed when the code was addapted
to python3.

'next' is needed, as it is used in other code parts, including
other related projects as taurus-pyqtgraph.

lib/taurus/core/util/containers.py

@@ -498,6 +498,8 @@ def __next__(self):
         self._index += 1
         return self.current()
 
+    next = __next__
+
     def previous(self):
         '''goes one item back in the list and returns it'''
         self._index -= 1