refactor: extract AnophelesDiversityAnalysis into anoph/Diversity.py

[Srivatsanray]

Overview

Relates to #366

This PR continues the anopheles.py refactoring effort by extracting the diversity analysis code into a dedicated module, following the same pattern used for AnophelesFstAnalysis, and AnophelesHetAnalysis.

Changes Made

New: Diversity.py

• Created AnophelesDiversityAnalysis class inheriting from AnophelesSnpData
• Moved 4 methods: _block_jackknife_cohort_diversity_stats, cohort_diversity_stats, diversity_stats and plot_diversity_stats

Modified: anopheles.py

• Removed the extracted methods
• Added AnophelesDiversityAnalysis to the inheritance chain

New: tests/anoph/test_diversity.py

• Added tests for cohort_diversity_stats, diversity_stats and plot_diversity_stats
• Parametrized across all 4 species simulators (ag3, af1, adir1, amin1)

Impact

• No changes to public API — all existing methods remain available via AnophelesDataResource
• Existing tests pass without modification, confirming MRO integrity
• 20 new tests all passing