site[s] convenient constructors for unit ranges/axes

[mtfishman]

For example, site("S=1/2") is now a shorthand for AbstractUnitRange(SiteType("S=1/2")), and sites("S=1/2", 4) is a shorthand for [AbstractUnitRange(SiteType("S=1/2")) for _ in 1:4]. These are generalizations of siteind and siteinds defined in ITensors.jl/ITensorMPS.jl.

Additionally, a unit range type can be passed as the first argument for customization, i.e.:

julia> using QuantumOperatorDefinitions: site

julia> site(UnitRange{Int32}, "S=1/2") === Int32(1):Int32(2)
true

The idea is that the next generation versions of siteind and siteinds (being developed in ITensor/ITensorMPS.jl#108) can just be defined as:

siteind(name::String; kwargs...) = site(Index, name; kwargs...)
siteinds(name::String, nsites::Int; kwargs...) = sites(Index, name, nsites; kwargs...)

where customizations like symmetry sectors and Hilbert space dimensions (in the case of bosons/qudits) can be passed as keyword arguments.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 69.50%. Comparing base (e08b46e) to head (65b4830).
Report is 1 commits behind head on main.

Additional details and impacted files

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+ Coverage   68.65%   69.50%   +0.85%     
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  Files          13       13              
  Lines         536      551      +15     
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+ Hits          368      383      +15     
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