Clustering capabilities for Lattice

[mofeing]

It would be interesting for Lattice to know that some tensors have been contracted together and keep track of them so that we can recover the same lattice. In order to do that, we need some kind of graph type where vertices can be grouped into clusters.

@mtfishman has this PartitionedGraphs.jl package that they use in ITensorNetworks.jl but I'm not sure it completely fits here. I would like it to be light, but that package seems to add a lot of things.