Changed VtkElement to not inherit from object.

Changed
-------
* VtkElement no longer inherits from builtins.object. This was needed
  because in Python 2.7 xml.dom.minidom.Element was not a new-style
  class. However, in Python 3.x it is, which causes a MRO error. Removed
  object from VtkElement since super is no longer used so it should be
  OK.