:return: <AbstractTreeNode> next node to process by a process with process_name

def get_next_node(self, process_name):
        """ :return: <AbstractTreeNode> next node to process by a process with process_name """
        if process_name not in self.process_hierarchy:
            raise ValueError('unable to compute the next_node due to unknown process: {0}'.format(process_name))

        time_qualifier = self.process_hierarchy[process_name].process_entry.time_qualifier
        return self._get_next_node(time_qualifier)