Self-directed edges for nodes in habitat

[rdnfn]

Description: it appears that the GeoGraph.add_habitat() method adds edges between nodes and themselves, but we probably don't want this as these edges don't have any meaning.

Example:
In the example here we have 2 nodes and 3 edges:

[herbiebradley]

Ah my bad, I think I prevented this in one version of the function but then it got removed somewhere along the way.

Since the fix is just to change if nbr in invalid_idx: to if nbr != node or nbr in invalid_idx: in the add edges loop, should I bother with a PR for this or just add a quick commit to the graph-analysis branch? Or maybe @Croydon-Brixton can batch it into his identification PR?

[rdnfn]

Ah right, thanks for getting back so quickly! There is no rush on my side, since it doesn't break any of my functions (they just draw zero length edges). So I am happy either way, quick commit or within the bigger PR. I wouldn't bother with a PR just for this commit.

[Croydon-Brixton]

if nbr != node or nbr in invalid_idx:

@herbiebradley @rdnfn
Added the fix in acc7874