Add option for unique contiguous numbering of vertices

[ko-phy]

The way the output is returned currently is well suited for LDPC codes, but suppose I just want to generate bipartite graphs, without a constraint / variable association. For this all vertices should be labelled uniquely. This would also be compatible with how graph libraries like igraph or networkx read graph data.

So if we have N vertices in the "left" part and M vertices in the "right" part, label indices should be unique integers in the range 0,...,M+N-1, where the first N correspond to "left" and the next M correspond to "right".

Even if one "forgets" that "left" / "right" vertex indices come first / last, the bipartition can be recovered in linear time.