Improve support for spatial and structural features, change SWP default

[Silmathoron]

This PR improves support for positions and Structure objects.
It fixes #148 by updating positions and structure upon node deletion.
It also corrects support for position and structure of from_matrix class method.

I also improved edge deletion with the default "nngt" backend.

Finally (unrelated) it also changes the default behavior of small_world_propensity to match normal behavior in the literature by using the average clustering coefficient instead of the global clustering.

[coveralls]

Coverage increased (+0.03%) to 72.581% when pulling 7cbf925 on delete-and-matrix into 6728231 on master.