refactor: extract AnophelesHetAnalysis into anoph/heterozygosity.py

- Move heterozygosity and ROH analysis methods from anopheles.py to
  new AnophelesHetAnalysis class in anoph/heterozygosity.py
- Implement _roh_hmm_cache_name property (fixes #900)
- Add ValueError guard for window_size > available sites in _sample_count_het
- Replace deprecated inplace=True in _roh_hmm_predict
- Add unit tests in tests/anoph/test_heterozygosity.py
Part of #366

Author: 31puneet