Adding Pf9 functionality

[nw20]

Adding a Pf9 class for accessing the Pf9 data release.

Changes/additions:

1. modify malariagen_data/__init__.py - add pf9 import
2. create malariagen_data/pf9.py - based on Pf7 template (no S3 mentions)
3. create malariagen_data/pf9_config.json - based on Pf8 template
4. modify malariagen_data/plasmodium.py - add 'low_memory = False' to csv import to prevent warnings
5. create tests/integration/test_pf9.py - based on Pf8 test template, adapted for new Pf9 metadata columns and expected length. Removed 'variant_altlen' from expected variant columns, it was not present in Pf8 and is not in Pf9 so I have removed rather than comment out. A note from Pf8 on this "Since it is possible to calculate this value (altlen) from REF and ALT, we are not concerned about it now. Therefore, this field is excluded from data_vars after reading the Pf8 zarr file."

Tests:

1. test_pf7, test_pf8, test_pf9.py all run and passed
2. test_plasmodium.py passed
3. run mypy malariagen_data tests --ignore-missing-imports no issues found
4. pre-commit hooks passed