Harden code and tests for robustness and memory management

[bbakernoaa]

This pull request introduces several robustness improvements, new cache management features, and expanded test coverage to the xregrid package. The main highlights include robust handling of edge cases in the regridding core, new methods for clearing driver and worker caches, improved error handling during ESMF regrid object creation, and comprehensive tests for lazy evaluation, cache isolation, and extreme coordinate values.

Robustness and Error Handling Improvements:

• The core regridding function now gracefully handles empty or all-NaN input arrays by returning arrays of the correct shape filled with NaNs, preventing crashes on edge cases.
• ESMF regrid object creation is now wrapped in try/except blocks in both parallel and non-parallel code, providing more informative error messages for common ESMF errors (e.g., out-of-range arguments, grid partition issues). [1] [2]

Cache Management Features:

• Added Regridder.clear_cache() and Regridder.clear_instance_cache() methods to allow manual clearing of global and instance-specific caches, both on the driver and on all Dask workers. The instance cache is also cleared automatically on object deletion. [1] [2] [3]
• Introduced _remove_from_worker_cache() utility to remove cache entries matching a pattern from worker-local caches.

Mask Handling:

• Improved mask extraction logic in _create_esmf_grid to correctly handle non-spatial dimensions by selecting the first slice, ensuring compatibility with conservative regridding.

Expanded and Improved Test Coverage:

• Added new tests for lazy evaluation (ensuring no premature computation), robustness to extreme coordinate values, cache isolation between multiple regridder instances, handling of empty and all-NaN input arrays, and cache clearing methods. [1] [2]
• Improved assertions in existing tests for clarity and consistency. [1] [2] [3] [4]

Documentation:

• Minor update to the README.md to clarify the package description.