Remove data variable inference API

[tomvothecoder]

Description

• Closes [FEATURE]: Remove data variable inference API #194

• It seems to be adding feature bloat and an unnecessary overhead when working with the API.
• By explicitly referencing the data var key, the API behavior remains consistent, predictable, and controllable.
  • For example, API behaves the same even if the dataset was opened via xarray or XCDAT, or if a single or multiple data vars exist
  • It might be redundant with a dataset that starts out with a single data variable, but if we add more data variables via agg methods (tas_climo), API behavior isn’t straightforward

• It cleans up our tests by eliminating the need to test the APIs with inferred data variables.

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 #196 (09045e3) into main (201a575) will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##              main      #196   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files            8         8           
  Lines          381       463   +82     
=========================================
+ Hits           381       463   +82     

Impacted Files	Coverage Δ
xcdat/__init__.py	100.00% <100.00%> (ø)
xcdat/axis.py	100.00% <100.00%> (ø)
xcdat/bounds.py	100.00% <100.00%> (ø)
xcdat/dataset.py	100.00% <100.00%> (ø)
xcdat/spatial_avg.py	100.00% <100.00%> (ø)
xcdat/xcdat.py	100.00% <100.00%> (ø)

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 194f84d...09045e3. Read the comment docs.

[tomvothecoder]

Just an FYI @jasonb5 and @pochedls.

@jasonb5 you may need to rebase and adjust your regridding API to remove inferred variables.
@pochedls Hopefully you can see why dropping this feature is a good idea from a software engineering perspective. The PR diff should communicate this as well.

[jasonb5]

@tomvothecoder Thanks, I'll adjust the regridding code.