refactor: normalize WellPlate.selected_wells

[tlambert03]

This PR changes the two of WellPlate.selected_wells from the very lax IndexExpression

Index = Union[int, List[int], Annotated[slice, _SliceType]]
IndexExpression = Union[Tuple[Index, ...], Index]

to a strict:

Tuple[Tuple[int, ...], Tuple[int, ...]]

The primary reason is that it makes checking equality between two plate plans easier.

it also modifies __repr_args__ to simplify the representation when possible

[codecov]

Codecov Report

Attention: Patch coverage is 93.93939% with 4 lines in your changes missing coverage. Please review.

Project coverage is 93.69%. Comparing base (7fd1744) to head (bed820b).

Files	Patch %	Lines
src/useq/_plate.py	93.93%	4 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #180      +/-   ##
==========================================
- Coverage   93.77%   93.69%   -0.08%     
==========================================
  Files          18       18              
  Lines        1188     1221      +33     
==========================================
+ Hits         1114     1144      +30     
- Misses         74       77       +3     

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