Iteration over sites for OrbitalSliceArray

[pablosanjose]

OrbitalSliceArrays are flat AbstractArrays{<:Number}, so iteration occurs over orbitals, not sites. We can also index them using a[sites(...)...] or view(a, sites[...]...) to get orbital blocks corresponding to stored sites, but often what we need is to simply iterate over all stored sites, and get all the blocks one after the other. We can currently do Quantica.cellsites(orbaxes(a)[1]) to get an iterator of CellSite indices that can be used to index a, but these are not exported.

Proposal

• sites(a::OrbitalSliceArrays, i) should be equivalent to Quantica.cellsites(orbaxes(a)[i])
• sites(a::OrbitalSliceArrays) should be a CartesianIndices iterator over all indices

[pablosanjose]

I had forgotten we have an exported siteindexdict(orbaxes(a)[1]) that gives a dictionary over CellIndices and flat indices.

It can be argued that sites means site positions in Quantica, so the above proposal would not be consistent. A more idiomatic interface would be o = orbaxes(a, 1) to be iterable, returning flat index ranges, i.e. the values of siteindexdict(o). Then we could do

for js in orbaxes(m, 2), is in orbaxes(m, 1)
  v = view(m, is, js)
  #do something with v
end

Note that orbaxes(a, n) = orbaxes(a)[n] would also need to be added.