`eachsiteinds` for lazy iteration over site indices of MPS/MPO

A common requirement in MPS and MPO code is to iterate over link or site indices (for example to prime them, etc.). It may be helpful to have a function that returns a lazy iteration over site indices or link indices, for example using a Base.Generator:

eachsiteinds(ψ) = (siteinds(ψ, n) for n in eachindex(ψ))
eachlinkind(ψ) = (linkind(ψ, n) for n in eachindex(ψ)[1:end-1])

s = siteinds("S=1/2", 4)
ψ = productMPS(s, "↑")
for s in eachsiteinds(ψ) @show s; end
for l in eachlinkind(ψ) @show l; end

returns:

s = IndexSet{1} (dim=2|id=317|"S=1/2,Site,n=1")
s = IndexSet{1} (dim=2|id=366|"S=1/2,Site,n=2")
s = IndexSet{1} (dim=2|id=155|"S=1/2,Site,n=3")
s = IndexSet{1} (dim=2|id=556|"S=1/2,Site,n=4")
l = (dim=1|id=986|"Link,l=1")
l = (dim=1|id=392|"Link,l=2")
l = (dim=1|id=691|"Link,l=3")