[GradedAxes] Introduce LabelledUnitRangeDual

[ogauthe]

This PR aims to fix cases where dual information is lost while slicing a BlockSparseArray. There are 2 cases:

• slicing a GradedUnitRangeDual with a Block now produces a (non-blocked) LabelledUnitRangeDual, which preserves dual information
• slicing a GradedUnitRangeDual with a vector of Block or BlockIndexRange produces a BlockVector whose only axis is a GradedUnitRangeDual. This is needed in views of BlockSparseArray.

[mtfishman]

Looks good to me, ready to merge from your end?

[ogauthe]

With this PR, we now have an error with

label(dual(labelled(1:1, "x")))

ERROR: MethodError: no method matching dual(::String)

Closest candidates are:
  dual(::NDTensors.SymmetrySectors.Ising)
   @ NDTensors ~/Documents/itensor/ITensors.jl/NDTensors/src/lib/SymmetrySectors/src/sector_definitions/ising.jl:24
  dual(::NDTensors.SymmetrySectors.Fib)
   @ NDTensors ~/Documents/itensor/ITensors.jl/NDTensors/src/lib/SymmetrySectors/src/sector_definitions/fib.jl:24
  dual(::NDTensors.SymmetrySectors.TrivialSector)
   @ NDTensors ~/Documents/itensor/ITensors.jl/NDTensors/src/lib/SymmetrySectors/src/sector_definitions/trivial.jl:15
  ...

Stacktrace:
 [1] label(a::LabelledUnitRangeDual{LabelledInteger{Int64, String}, LabelledUnitRange{Int64, UnitRange{Int64}, String}})
   @ NDTensors.GradedAxes ~/Documents/itensor/ITensors.jl/NDTensors/src/lib/GradedAxes/src/labelledunitrangedual.jl:16
 [2] top-level scope
   @ REPL[12]:1

Maybe we should default dual(x) = x.

[mtfishman]

Maybe we should default dual(x) = x.

That would make sense as a definition, we do that as a default definition for unit ranges anyway.

[ogauthe]

Ready to merge!

[mtfishman]

Ready to merge!

Great, thanks. I'll merge once tests pass.