Add a test for boundary_angles in topology.py

[JulienLefevreMars]

It uses the function create_K_rings that builds k rings with k from 1 to K-1

On each layer k, all the points (6k) are regularly sampled on a circle of radius k and angles pi / (3 k ). Therefore the angle between consecutive sides is pi - pi/(3k) which yields the unit test.

[JulienLefevreMars]

Thanks for the merge @alexpron . I need to do a personal update regarding what the docstrings are for the future.