Refactor: Reduce code duplication and cache country region checks

[prantogg]

Cleans up core.py by removing duplicated code and adding a simple performance optimization.

Changes:

• Dynamic method delegation - ChainableDataFrame now delegates to Wkl via __getattr__ instead of 11 separate wrapper methods
• Cache _has_region checks - Store results in a module-level dict to avoid repeated SQL queries for the same country
• Extract helper methods - _get_subdivisions() for counties/cities
• Simplify conditionals - Remove redundant branches in regions(), counties(), cities()