[ENHANCEMENT] Define LinearAlgebra.diag and LinearAlgebra.diagind for ITensors

[mtfishman]

Define overloads of LinearAlgebra.diag and LinearAlgebra.diagind for ITensors to make it easier to work with diagonal entries (iterate, modify, etc.). A corner case will be figuring out how to handle the block sparse case in a good way (i.e. only iterating over the diagonals of nonzero diagonal blocks, if that is the desired behavior).

Note that conversion to Tensor can help with manipulating diagonal entries:

julia> using ITensors, ITensors.NDTensors, LinearAlgebra

julia> i = Index(2)
(dim=2|id=887)

julia> A = randomITensor(i', i);

julia> @show A;
A = ITensor ord=2
Dim 1: (dim=2|id=887)'
Dim 2: (dim=2|id=887)
Dense{Float64, Vector{Float64}}
 2×2
  1.9193797093690799  0.16138987630693938
 -1.141571133623248   0.43841019457419206

julia> d = diag(tensor(A))
Dim 1: 2
Dense{Float64, SubArray{Float64, 1, Vector{Float64}, Tuple{StepRange{Int64, Int64}}, true}}
 2-element
 1.9193797093690799
 0.43841019457419206

julia> for i in 1:length(d)
         d[i] = d[i]^2
       end

julia> @show A;
A = ITensor ord=2
Dim 1: (dim=2|id=887)'
Dim 2: (dim=2|id=887)
Dense{Float64, Vector{Float64}}
 2×2
  3.6840184687377335  0.16138987630693938
 -1.141571133623248   0.19220349870658093