Provide a function to generate NetworkGraph

[PabloCastellano]

The _netjson_networkgraph function is used to generate a NetJSON NetworkGraph.

https://github.com/openwisp/netdiff/blob/master/netdiff/utils.py#L127

Right now it is more like an internal function. It would be nice providing it to the module users so that they don't have to reinvent the wheel so much.

For example, in openwisp-network-topology, the json method of the Topology model is already reinventing it:

https://github.com/openwisp/openwisp-network-topology/blob/fac83331cee7194aef4ba92f0a5fa067d3597aac/openwisp_network_topology/base/topology.py#L139-L169

[nemesifier]

I'm not sure I see the benefit in doing this. Creating a NetworkGraph JSON object is pretty easy and the way it's constructed depends on the application internals.