feat: add list_taxons and list_species_calls utilities to SampleMetadata

[Aadik1ng]

This PR addresses a common user need for simpler taxon and species discovery within sample sets. Currently, discovering which unique taxonomic groups are present in a release requires manually loading all sample metadata and computing unique values.

The new methods list_taxons() and list_species_calls() provide a clean, read-only API for this common task, improving the overall user experience for data exploration and preparation.

Changes

• Added AnophelesSampleMetadata.list_taxons(): Returns a sorted list of unique taxon names.
• Added AnophelesSampleMetadata.list_species_calls(): Returns a sorted list of unique species calls from AIM data (if available).
• Updated malariagen_data/anoph/sample_metadata.py with robust implementations that handle missing columns and data types gracefully.
• Added comprehensive numpydoc-compliant docstrings with usage examples.

Testing

Added new test cases to tests/anoph/test_sample_metadata.py:

• test_list_taxons
• test_list_species_calls

Verified functionality across the full suite of simulated datasets:

• ag3
• af1
• adir1
• amin1

All 20 newly added test cases passed locally with 100% success.

[Aadik1ng]

@jonbrenas Can you please review this pr?

[jonbrenas]

Thanks @Aadik1ng, I am not sure list_species_calls is a very accurate name. Wouldn't list_aim_calls or list_aim_species_calls be better?

I think there is a duplicated return in your code.