Remove XCDATAccessor class

[tomvothecoder]

Description

• Closes [FEATURE]: Consider removing the XCDATAccessor class to avoid redundancy #218

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 #222 (47c3003) into main (1fdc8a9) will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##              main      #222   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files            9         8    -1     
  Lines          742       693   -49     
=========================================
- Hits           742       693   -49     

Impacted Files	Coverage Δ
xcdat/__init__.py	100.00% <ø> (ø)
xcdat/utils.py	100.00% <ø> (ø)
xcdat/spatial.py	100.00% <100.00%> (ø)

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[tomvothecoder]

Hey @pochedls and @jasonb5, I rolled out this update pretty quickly.

Not really much to review, just deleted some files, update the API docs main page, and renamed .spatial_avg() to .spatial(). Good to see if you have any thoughts though.