Allow indexing arrays with states

[lpawela]

This allows us to index arrays with states, eg

σ = (1, -1, -1, 1, 1)
d = 2^length(σ)
x = rand(2, 2, d)
@state x[1, 1, σ] # == x[1, 1, 20]

[kdomino]

But how does it comply with the (low) energy spectrum, where we can not map: (-1, .., -1, -1) -> 1, (-1, .., -1, 1) -> 2 etc ...

[bartekGardas]

I do not understand @kdomino objection. Can you please elaborate a bit more? The 1-1 mapping do exist, and hence it can be used to establish "state numbering".

[kdomino]

In the low energy spectrum you sort states by energies, hence indexing is energy dependent this is mode 5, am I right?
In modes 1-4 you may number (sub)states as you wish, i.e. either using subspace of the energy spectrum or simple mapping (-1, .., -1,-1) -> 1; (-1, ..., -1, 1) -> etc .... Nevertheless It must be consistent, i.e. in PEPS building and in setting particular index value.

[kdomino]

Non working example is in branch kd/indexing-tests benchmarks/indexing_example.jl

[kdomino]

https://github.com/iitis/SpinGlassPEPS.jl/tree/kd/indexing-tests/benchmarks

[bartekGardas]

update_cells!(
g_ising,
rule = square_lattice((m, 1, n, 1, t)),
)

[bartekGardas]

It should work for now. Let's add some examples, especially for comparison with the Gibbs state.

[bartekGardas]

Let's merge as is, it should work for now.