Add ._copy method and _bounds attr to DataArrayBoundsAccessor

[tomvothecoder]

Description

This PR removes adding the bounds dimension and bounds variables (as coordinates) to the DataArray in DataArrayBoundsAccessor. Instead, a separate _bounds dataset stores the bounds. The advantage of using a dataset is that we can leverage methods of cf_xarray and DatasetBoundsAccessor.

To retain the attributes of an accessor class when copying a variable, each class must have a .copy() method to return a copy of the xarray data structure with cached attributes. There is also a copy_variable() function to handle this for the user so they don't have to invoke each of the individual .copy() methods themself.

• Closes Unexpected behavior with open_variable #97

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]

Codecov Report

Merging #98 (f359dd5) into main (c9d0529) will not change coverage.
The diff coverage is 100.00%.

❗ Current head f359dd5 differs from pull request most recent head 4d40673. Consider uploading reports for the commit 4d40673 to get more accurate results

@@            Coverage Diff            @@
##              main       #98   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files            5         5           
  Lines          180       163   -17     
=========================================
- Hits           180       163   -17     

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

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[tomvothecoder]

Added this comment for developers in case this confuses them on why a Dataset is being stored inside a DataArray accessor class (think of it as just a container, like a dict).

[tomvothecoder]

Hey @pochedls, I just finished my review. Let me know how things look, and I'll merge so we can move forward with #87/#95.

[pochedls]

@tomvothecoder - looks good to me! I noticed that within ts.bounds._bounds some of the axis arrays are repeated (e.g., lat is in this dataset in addition to lat_bnds even thought lat is already a part of the dataarray via ts.lat). I think I understand why this is and have no issue with this approach.