Method retrieves a tree node identified by the time_qualifier and the timeperiod

def get_node(self, process_name, timeperiod):
        """ Method retrieves a tree node identified by the time_qualifier and the timeperiod """
        if process_name not in self.process_hierarchy:
            raise ValueError('unable to retrieve the node due to unknown process: {0}'.format(process_name))

        time_qualifier = self.process_hierarchy[process_name].process_entry.time_qualifier
        return self._get_node(time_qualifier, timeperiod)