Replace __getattr__ with explicit collection interface in Dimensions

[pnorton-usgs]

Replace getattr with explicit collection interface in Dimensions

Summary

Removes the implicit __getattr__ delegation to the internal dict in Dimensions and replaces it with explicit, documented collection methods. This makes the class interface discoverable, type-checkable, and prevents accidental mutation of internal state.

Motivation

The previous __getattr__ silently forwarded any unknown attribute access to the underlying dict, which:

• Exposed mutating methods (.pop(), .clear(), .update()) that bypass validation logic
• Broke IDE autocomplete and static analysis (mypy couldn't see available methods)
• Produced confusing error messages on typos (errors came from dict, not Dimensions)
• Required a special __setstate__ guard to avoid pickling issues

Changes

• Remove __getattr__ delegation to internal self.__dimensions dict
• Add __contains__ — enables 'nhru' in dims syntax
• Add __iter__ — enables for name in dims: iteration
• Add __len__ — enables len(dims)
• Add explicit keys(), values(), items() methods preserving the existing read-only API used throughout the codebase
• Update exists() to use __contains__ internally
• Update ndim property to use __len__ internally

Testing

All 290 tests pass. No behavioral changes for existing callers — dims.keys(), dims.values(), and dims.items() continue to work as before.