[ITensors] Add a debug check that sets of Index are unique

[mtfishman]

Right now, this is allowed, even in debug mode:

julia> i = Index(2)
(dim=2|id=241)

julia> is = IndexSet(i, i)
IndexSet{2} (dim=2|id=241) (dim=2|id=241) 

julia> ENV["JULIA_DEBUG"] = "ITensors"
"ITensors"

julia> is = IndexSet(i, i)
IndexSet{2} (dim=2|id=241) (dim=2|id=241) 

This could be done in the IndexSet constructor, with the Julia function allunique, i.e. just add @debug allunique(data) || error("Indices must all be unique in IndexSet") here.