Update randgraphs.jl

[javier-gallardo-saenz]

Added Hierarchical Configuration Model to randgraphs.jl

I have tested its performance for small/medium numbers of nodes and communities, but further testing might be necessary.

This implementation allows self-loops and multiple edges (which are reduced here to a simple edge), so the resulting degree sequences might not exactly matched the specified sequences. Thus, I am wondering if this function might be better suited for Multigraph.jl, but since the repository includes the Configuration Model I figured it would make sense that it included the Hierarchical Configuration Model as well (as this is basically its extension to community structure).

Some external degree sequences cannot be realized even if they add up to an even number. To detect such sequences is not trivial, so when a external degree sequence is 'correct' (adds up to even number) but cannot be realized the function should return the closest possible realization.

Best regards,
Javier