Code test coverage needed in base.py

[leehart]

Code test coverage report: https://app.codecov.io/gh/malariagen/malariagen-data-python/commit/4c9100b1353e33d92fcc57837450c680f547812c

Appears to mostly concern a 16% drop in test coverage in anoph/base.py, mostly around the recently added property functions relating to the unrestricted data and surveillance flags, e.g. _releases_with_unrestricted_data, _releases_with_surveillance_data, _relevant_releases, _release_has_surveillance_data, _sample_set_has_surveillance_data, _sample_set_has_unrestricted_use, etc.