New QN ITensor constructors

[mtfishman]

It would be nice to have QN ITensor constructors that can make multiple blocks at once, instead of block-by-block with setindex! for all possible blocks by specifying the flux. For example:

i = Index(QN(0) => 2, QN(1) => 2; tags="i")
j = Index(QN(0) => 2, QN(1) => 2; tags="j")
ITensor([Block(1,1), Block(2,2)], i, dag(j))  # Create ITensor with nonzero blocks (1,1) and (2,2)
ITensor([(i => 1, j => 2, 12), (i => 4, j => 4, 44)], i, dag(j)) # Create ITensor with nonzero blocks (1,1) and (2,2) and the specified elements set

Also, it would be nice to think about an interface for indexing within a block (i.e. instead of (i => 4, j => 4), set the (2, 2) element of block (2, 2)), like the BlockIndex interface discussed here: ITensor/NDTensors.jl/issues/9

[mtfishman]

An alternative to the second constructor could be a special way of setting elements:

T = emptyITensor(i, dag(j))
T[[(i => 1, j => 2), (i => 4, j => 4)]] .= [12, 44]

or with shorter notations:

T[(i, j) => [(1, 2), (4, 4)]] .= [12, 44]
T[[(1, 2), (4, 4)]] .= [12, 44]