Try to use petgraph::csr as an internal representation for a Network

[adinapoli-mndc]

At the moment we define:

pub struct Network {
    from_graph: petgraph::Graph<Artifact, Dependency, Directed>,
}

However the Graph types uses adjacency lists under the hook, whereas our graph is likely to be sparse for the most parts. petgraph offers a petgraph::csr::Csr variant, but it doesn't seem to be a drop-in replacement, so this has to be investigated.

[adinapoli-mndc]

After some more investigation, I am not sure this can be easily achieved, as the API seems to be quite different between the two implementation. add_edge, for example, returns an EdgeIndex for the Graph API, but just a bool for the Csr one.

[adinapoli-mndc]

I feel this is now a dupe of #29, so I am going to close it.