Restore GradedUnitRange slicing

[mtfishman]

Restore some GradedUnitRange slicing that was dropped in #79. Split off from #129.

[codecov]

Codecov Report

❌ Patch coverage is 90.47619% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 62.14%. Comparing base (2dddcde) to head (a97d09b).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
src/gradedarray.jl	86.95%	3 Missing ⚠️
src/sectorarray.jl	50.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #130      +/-   ##
==========================================
+ Coverage   59.95%   62.14%   +2.18%     
==========================================
  Files          10       10              
  Lines         849      869      +20     
==========================================
+ Hits          509      540      +31     
+ Misses        340      329      -11     

Flag	Coverage Δ
docs	0.00% <0.00%> (ø)

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