NetworkX graphs to RDFLib graphs

Now there are only [functions of converting from RDFLib graphs to NetworkX graphs](https://github.com/RDFLib/rdflib/blob/d717cef505b07556fffc9f0e64d6c8f5b3a5b84c/rdflib/extras/external_graph_libs.py).

If there are actually no functions to convert RDFLib graphs to NetworkX graphs, then we can discuss how to add them in this issue?