[ITensors] [QN] Promote QNs when constructing ITensors or creating IndexSets

[mtfishman]

Right now, an IndexSet or ITensor can be created such that the indices do not all need the same QNs, for example:

i = Index([QN(("Sz",1),("N",0))=>2,QN(("Sz",2),("N",1))=>1],"i")
j = Index([QN("Sz",1)=>2,QN("Sz",2)=>1],"j")

A = randomITensor(QN(),i,dag(j))

It may be good for efficiency to promote all of the indices such that they have the same QNs (filling the missing ones with QNs with zero values). Then, functions can assume all of the QNs exist, and operations between QNs like addition and equality are faster.