add graph stat betweeness and test case

[kyao]

[pep8speaks]

Hello @kyao! Thanks for updating this PR. We checked the lines you've touched for PEP 8 issues, and found:

• In the file tests/test_1graph_statistics.py:

Line 33:9: E303 too many blank lines (2) Line 65:5: E303 too many blank lines (2)

Comment last updated at 2022-08-12 03:40:46 UTC