GraphVector struct

[kunyuan]

Implement a data structure to handle vector graphs:

NOTE: All graphs in the vector should have the same number of external legs, but the labels of the external legs can be different.

• Add the following struct
  struct GraphVector{F, W} <: AbstractVector
  graphs::Vector{Graphs{F, W}}
  end
• Implement vector indexing through julia iterator API
• A function to group the graphs with the same external legs with a given index. For example,

For a GraphVector, gv = GraphVector([g(1, 2), g(1, 3), g(2, 3)]). We expect the following:

1. merge(gv, indices = [1, ]) = [GraphVector([g(1, 2), g(1, 3)]), GraphVector([g(2,3), ])]
2. merge(gv, indices = [2, ]) = [GraphVector([g(1, 2), ]), GraphVector([g(2,3), g(1, 3)])]
3. merge(gv, indices = [1, 2]) = [GraphVector([g(1, 2), ]), GraphVector([g(2,3), ]), GraphVector([g(1, 3), ])]

• construct Feynman diagram from a set of GraphVectors.

function feynman_diagram(vertices::Vector{GraphVector}, topology::Vector{Vector{Int}};
external=[], factor=one(_dtype.factor), weight=zero(_dtype.weight), name="", type=:generic)