Rename accessor classes and methods for API consistency

[tomvothecoder]

Description

The goal of this PR is to make the API naming convention consistent across accessor classes.

The Dataset portion of accessor class names are removed because it is redundant.

• Closes Align xcdat.XCDATAccessor public API names with other accessor classes #140
• Rename DatasetBoundsAccessor to BoundsAccessor (breaking change)
• Rename DatasetSpatialAverageAccessor to SpatialAverageAccessor (breaking change)
• Rename SpatialAverageAccessor.avg() to SpatialAverageAccessor.spatial_avg() (breaking change)
  • The original reason why it was called .avg() was because the accessor class and decorator (ds.spatial.avg) already describes what the averaging operation was targeting. However, it was not aligned with ds.xcdat.spatial_avg(), which can throw users off.
• Add tests for SpatialAverageAccessor class
• Add accessor classes to xcdat.__init__.py
• Remove flake8 noqa comments in xcdat.py

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) -- won't increment as a major release, just note in v0.2.0 changelog

[tomvothecoder]

Hi @pochedls, it'd be great if you can review this PR so you're aware of the changes since it affects the spatial averaging API. Thanks!

[pochedls]

No objections to the API name changes (I think this is a good idea).

[lee1043]

I second @pochedls!

[tomvothecoder]

Awesome, thanks @pochedls and @lee1043