Add expand.bipartite= argument to `[.network()` and `[<-.network()` methods?

[krivit]

Per #94 (comment) , the nw[i,j] != as.matrix(nw, matrix.type = "adjacency")[i,j]. You can make it true by passing expand.bipartite = TRUE to as.matrix(). Would it make sense to provide an expand.bipartite= argument to the [ methods to allow it to work in the opposite direction as well?