Update temporal.py to properly handle piControl simulations

[lee1043]

Description

PiControl simulations can have time step labels as like "0001-01-01". To not lose the first "000" in year, #695 proposed this fix.

• Closes [Bug]: Potential bug in temporal.py module #695
• Convert self.dim in TemporalAccessor from Hashable to a string to fix mypy warning and remove type ignore comments

Checklist

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• My changes generate no new warnings
• Any dependent changes have been merged and published in downstream modules

If applicable:

• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass with my changes (locally and CI/CD build)
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• I have noted that this is a breaking change for a major release (fix or feature that would cause existing functionality to not work as expected)

[lee1043]

pre-commit shows following error for mypy failing, which I don't have a good idea. @tomvothecoder can you help on this?

$ pre-commit run --files temporal.py 
trim trailing whitespace.................................................Passed
fix end of files.........................................................Passed
check yaml...........................................(no files to check)Skipped
black....................................................................Passed
isort....................................................................Passed
flake8...................................................................Passed
mypy.....................................................................Failed
- hook id: mypy
- exit code: 1

xcdat/temporal.py:1288: error: Argument 1 to "groupby" of "DataArray" has incompatible type "Any | Hashable"; expected "str | DataArray | IndexVariable | Sequence[Hashable] | Mapping[Any, Grouper] | None"  [arg-type]
Found 1 error in 1 file (checked 1 source file)

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 100.00%. Comparing base (753a046) to head (9669dfc).
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff            @@
##              main      #696   +/-   ##
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  Coverage   100.00%   100.00%           
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  Files           15        15           
  Lines         1546      1546           
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  Hits          1546      1546           

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[tomvothecoder]

I fixed the mypy warning that was appearing with the latest version of xarray. This PR looks good to me. Thank you Jiwoo.

[lee1043]

@tomvothecoder thank you for your review! I am merging it now.