Simplify bra, operator, and ket vertex logic of form networks with partitioned graphs

[mtfishman]

I think that it could be simpler to implement the bra, operator, and ket vertex logic in form networks with two layers of partitioned graphs. The outer layer would handle the BP-like partitioned, and then each of those partitions could be partitioned into bra, operator, and ket vertices.

@JoeyT1994

[JoeyT1994]

Yeah I think it is worth seeing what a re-definition of the FormNetwork type as just a multi-layered PartitionedGraph would bring us. It might make things even easier to reason about.