code: def _get_argument(self, argument_node):
          
          argument = FritzActionArgument()
          argument.name = argument_node.find(self.nodename('name')).text
          argument.direction = argument_node.find(self.nodename('direction')).text
          rsv = argument_node.find(self.nodename('relatedStateVariable')).text
          # TODO: track malformed xml-nodes (i.e. misspelled)
          argument.data_type = self.state_variables.get(rsv, None)
          return argument
docs: Returns a FritzActionArgument instance for the given argument_node.