Replace "time" references with self._dim in class TemporalAccessor

[tomvothecoder]

Description

This PR updates incorrect static references to "time" as the name of the time dimension with the dynamic self._dim dimension name.

• Closes [Bug]: Replace static references to "time" for the time dimension name with self._dim in TemporalAccessor #311

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)

[codecov-commenter]

Codecov Report

Merging #312 (d60649b) into main (4c51879) will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##              main      #312   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           14        14           
  Lines         1170      1170           
=========================================
  Hits          1170      1170           

Impacted Files	Coverage Δ
xcdat/temporal.py	100.00% <100.00%> (ø)

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[tomvothecoder]

Related #285, we might need to update how we get the dimension name for a time axis (self._dim) in TemporalAccessor().

Instead of getting the time dimension name from the dataset (which can have multiple time axis):

xcdat/xcdat/temporal.py

Line 149 in 4c51879

self._dim = get_axis_coord(self._dataset, "T").name

We can get just the single time axis for the data variable that is being temporally averaged:
self._dim = get_axis_coord(self._data_var, "T").name