Define simpler `infsiteinds` that shifts the flux density to zero

[mtfishman]

Define a function like:

infsiteinds(initstate, sitetype, nsites; kwargs...)

where initstate is a function defining a product state, for example:

initstate(n) = isodd(n) ? "↑" : "↓"

which we will use internally to determine the flux density (flux of the unit cell) and shift the quantum numbers accordingly.