New Multiplicity{M} type

[brainandforce]

This type should facilitate indexing of some new data types by spin state. SpinRange{M} has a single type parameter, the multiplicity M, which is an Int, and it behaves like UnitRange(-(M-1)//2, (M-1)//2), where all elements are valid angular momenta for that multiplicity.

I am willing to go back and rename this type to Multiplicity or something else, because even though SpinRange is an AbstractUnitRange, the type parameter M is the spin multiplicity.

[brainandforce]

I decided to rename the type to Multiplicity{M} and rebased it on main to add the test fixes from the previously merged PR.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (1311875) 59.77% compared to head (b535f66) 58.69%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #199      +/-   ##
==========================================
- Coverage   59.77%   58.69%   -1.09%     
==========================================
  Files          20       21       +1     
  Lines        1432     1346      -86     
==========================================
- Hits          856      790      -66     
+ Misses        576      556      -20     

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