Extract method _postprocess_dataset() and make bounds generation optional

[tomvothecoder]

This PR extracts the _postprocess_dataset() function from xcdat.open_dataset() and xcdat.open_mfdataset() to reduce code duplication.

It also adds the optional boolean kwarg add_bounds (defaults to False) to both dataset opening functions, since generating missing bounds might not be desired in some cases. If the user ends up needing to generate bounds, they can either call ds.bounds.add_missing_bounds() to generate bounds that are missing for all supported axis (time, lat, lon), or ds.bounds.add_bounds("key_of_coords") for a specific axis.

Summary of Changes

• Add boolean kwarg add_bounds for adding bounds to dataset opening functions, defaults to False.
• Update docstrings to reduce redundancy and increase conciseness
• Update variable names for clarity
• Update reference to "regular" data vars to "non-bounds" data vars
• Update optional function arg data_var to required arg key
• Update line spacing and make sure there is a line break with return statements
• Update BoundsAccessor property names for clarity

Description

• Closes [FEATURE]: Make generating bounds optional in open_dataset() and open_mfdataset() #220

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 #223 (8c78cf1) into main (1fdc8a9) will not change coverage.
The diff coverage is 100.00%.

❗ Current head 8c78cf1 differs from pull request most recent head 89ad615. Consider uploading reports for the commit 89ad615 to get more accurate results

@@            Coverage Diff            @@
##              main      #223   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files            9         8    -1     
  Lines          742       700   -42     
=========================================
- Hits           742       700   -42     

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

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[tomvothecoder]

Update docstrings