Error with Julia 1.6.0

[davide-f]

Dear @oxinabox ,

while using SankeyPlots, I discovered an error in the inizialization of the variable T in the function ordering_problem, when I updated Julia to the 1.6.0 version with the lastest packages.
In particular, the following error is thrown:
TypeError: in DenseAxisArray, in L, expected L<:Tuple{Vararg{JuMP.Containers._AxisLookup, N}}, got Type{Tuple{Dict{Int64, Int64}}}

The problem is related to: line 108 of zarate.jl function:
T = JuMP.Containers.DenseAxisArray{VariableRef,1,Tuple{Vector{Int64}},Tuple{Dict{Int64,Int64}}}

[davide-f]

I have solved the issue; I add a pull request