fix graph tool segmentation fault

[lrnzgiusti]

This pull request fixes a SIGSEGV from graph-tool=2.44 when calling gt.stats in function cwn.data.utils.get_rings

as noted in graph_tool doc:

# import numpy and scipy before everything to avoid weird segmentation faults
# depending on the order things are imported.

I haven't run an exhaustive debugging but importing numpy or torch before graph_tool leads to a segfault while computing the rings of a graph.

This code will reproduce the error:

After changing the position of import graph_tool as gt

(I set the matrix-style terminal many years ago and never changed, sorry for that ahah)

[crisbodnar]

Thanks for the PR! Merged.