NNGT 2.5.0: sparse clustered graphs and improved backend support

This new version adds support for NetworkX 2.6+, introduces a new network model, improves IO and plotting modules, and extends the list of functions that are supported by the default backend (if networkx, igraph, and graph-tool are not installed on the system).

New features

• sparse_clustered: it is now possible to generate sparse graphs with a desired clustering coefficient, this comes with an associated rewiring scheme (f45d0a9b)
• support for GraphML file format (a9f8710d)
• support NetworkX 2.6+ (f4e5002e)

Enhancements

• More efficient edge attribute access (speed and memory improvements, c8f29951)
• Improved plots (color and size support for nodes and edges, visible arrows for directed networks..., 7f5340b8 & ccdd627f)
• Add connectedness test with nngt default backend (d3979a15)

Bugfixes

Correct delete_edges for nngt backend that did not update the node degrees (b1ca08df).