Returns:
A list of edge groups and a list of corresponding node groups,
- where the group is a list of IDs of edges or elements, like follows
+ where the group is a list of IDs of edges or nodes, like follows
[[[branch_edges_1],[branch_edges_2]], [[branch_nodes_1],[branch_nodes_2]]].
If a group is closed, the first and last nodes of the group are same.
"""
