Make types of fields in `PartitionedGraph` more specific

[mtfishman]

The partitioned_vertices and which_partition fields of PartitionedGraph are currently abstract: https://github.com/mtfishman/NamedGraphs.jl/blob/d561823168be7d042fcfac31162c6c2495f10ba4/src/Graphs/partitionedgraphs/partitionedgraph.jl#L5-L6. To improve performance and type stability they should be parametrized, see https://docs.julialang.org/en/v1/manual/performance-tips/#Avoid-fields-with-abstract-type. @JoeyT1994

[JoeyT1994]

So you mean like:

struct PartitionedGraph{V,PV,G<:AbstractGraph{V},PG<:AbstractGraph{PV}, PV<:AbstractDict, WP<:AbstractDict} <:
       AbstractPartitionedGraph{V,PV}
  graph::G
  partitioned_graph::PG
  partitioned_vertices::PV
  which_partition::WP
end

instead?

[mtfishman]

I was thinking like this:

struct PartitionedGraph{V,PV,G<:AbstractGraph{V},PG<:AbstractGraph{PV}} <:
       AbstractPartitionedGraph{V,PV}
  graph::G
  partitioned_graph::PG
  partitioned_vertices::Dictionary{PV,V}
  which_partition::Dictionary{V,PV}
end

which restricts the storage to be Dictionary but either style fixes the type stability issue. Basically the point is that you should try to expose the types of the fields as much as possible in the type parameters of PartitionedGraph (there are reasons for not doing that if you want the object to be more dynamic, like the ITensor type, but that is fairly uncommon and there isn't a good reason for doing that here).