Remove center_times kwarg from temporal averaging methods

[tomvothecoder]

Description

• Closes [Feature]: Remove center_times from temporal averaging methods #253
• Move center_times() from class TemporalAccessor to test_axis.py as a module-level function
• Add private function _get_coord_var() for fetching coordinate var using CF-compliant axis attr with cf_xarray
• Remove redundant private tests in test_spatial.py
  • Related to [FEATURE]: Refactor test suite that covers private tests #242

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)