Define sector_type

[ogauthe]

This PR introduces a new function sector_type as a high level method to access the sector type without relying on implementation details such as the existence of LabelledNumbers.jl. The objective is to use GradedUnitRanges.sector_type as a generic method to access sector for various types:

GradedUnitRanges.sector_type(S::Type{<:AbstractSector}) = S
GradedUnitRanges.sector_type(F::Type{<:FusionTree}) = ...
GradedUnitRanges.sector_type(FT::Type{<:FusionTensor}) = ...

To be decided:

• currently sector_type errors for non-labelled axes. It may instead return NoLabel(), or a custom struct.
• sector_type could also be defined inside SymmetrSectors, but I think it makes more sense here.

[ogauthe]

The tests are failing now due to an issue in LabelledNumbers. They will pass with ITensor/LabelledNumbers.jl#7.

[mtfishman]

Thanks!