add utility fusiontreetype

Author: lkdvos

src/fusiontrees/fusiontrees.jl

@@ -145,6 +145,9 @@ function fusiontreetype(::Type{I}, N::Int) where {I<:Sector}
         FusionTree{I,N,N - 2,N - 1}
     end
 end
+function fusiontreetype(::Type{I}, N₁::Int, N₂::Int) where {I<:Sector}
+    return Tuple{fusiontreetype(I, N₁),fusiontreetype(I, N₂)}
+end
 
 # converting to actual array
 function Base.convert(A::Type{<:AbstractArray}, f::FusionTree{I,0}) where {I}